Chemistry of inorganic compounds – Carbon or compound thereof – Binary compound
Reexamination Certificate
1999-12-14
2002-09-10
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Binary compound
Reexamination Certificate
active
06447742
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention involves methods for production of fine-grain metal carbide powders and the resulting fine-grain powders and more particularly to tungsten carbide (WC) powder.
2. Background Information
Since the early days of industrial cemented carbide production, tungsten carbide (WC) powder has remained an important starting material. For decades tungsten oxide powder or powder of a tungsten oxide-containing compound has been filled into boats for its production, reduced under a reducing atmosphere for more than 10 hours to tungsten powder, the tungsten powder then mixed with carbon powder and again filled into a boat and made to react to tungsten carbide during a process lasting several hours by carbon diffusion into the tungsten grains. Tungsten carbide powder with particle size lower limits of a few &mgr;m diameter were produced according to the technically available particle sizes for the tungsten (W) metal obtained by reduction of the oxide.
With further development of improved cemented carbide grades over the past two decades and intensified demand for so-called submicron tungsten powder grades, gas carburizing methods have gained significance in addition to carburizing by carbon diffusion. As the name suggests, carburizing of tungsten in the gas carburizing method occurs by means of carbon transported via the gas phase to the tungsten surface. Methods are also known in which tungsten carburizing occurs both via solid carbon added to the tungsten powder and via carbon-containing gases. A significant reduction in process time and simplification of process control is expected with the application of the gas phase reaction, especially because undesired depositing of free carbon appears to be thermodynamically manageable via control and metering of the gas phase. It should also be expected that in the gas carburizing method, depending on the process temperature, particle enlargement in the already formed WC can be avoided or kept within tolerable limits. Particle enlargement in the already formed WC is unavoidable in the diffusion process as a result of longer process times.
Gas carburizing processes occur in a fluidized bed, ordinarily in rotary tube kilns or fluidized bed chambers. The corresponding installations operate continuously with respect to the product being carburized or in a sequence with stipulated filling amounts.
Coarse-grained dense tungsten carbide powder cannot be technically reduced or ground to powders with a particle size <1 &mgr;m. These are so-called submicron powders. It would therefore seem appropriate to arrive at submicron tungsten carbide powder using another technique. One such technique is by using submicron initial powders as the product being carburized via the gas carburizing process.
However, implementing a gas carburizing process for submicron WC powders is not easy. In fact, when the above ideas were implemented, a number of difficulties appeared that find expression in a large number of different proposals for process control.
Among the process developments for production of submicron or nanocrystalline tungsten carbide powder or WC—Co powder by gas carburizing, mostly two methods have been proposed, and will be further described below as representative with reference to U.S. Pat. No. 5,372,797 (the '797 patent) and European Pat. No. EP B 0 598 040 (the '040 patent).
A process for production of tungsten carbide with a particle size of 50 to 200 nm is described in the '797 patent. A solid tungsten-containing starting material is allowed to react in a flowing gas atmosphere containing hydrogen and molecular methane “until nearly all the tungsten starting material is converted to WC.” The reaction occurs in two process sections, a first at 520° to about 550° C. and a second with a heating rate of 3° to 10° C. per min at about 800° to about 900° C. The temperature rise and partial pressure of the forming water is controlled so that “the mentioned powder particle size is produced.” According to the main claim, at least 50% of the tungsten carbide with a particle size of 50 to 200 nm is to be formed during at least 15 min of reaction time in the second process section. The tungsten carbide so produced has a BET surface from 1 to 10 m
2
/g, whereas the tungsten-containing starting material should possess a BET surface from 0.01 to 0.09 m
2
/g, but at the same time commercial “GTE TO3” tungsten carbide powder with an average particle size of about 10 to 30 &mgr;m is described. According to Example 1 of the '797 patent, the reaction gas consists of 3% molecular methane and 97molecular hydrogen. The first partial reaction occurs after heating at a heating rate of 20° C. per min to 535° C. process temperature, the second reaction during a process temperature of 850° C. maintained for 90 min, which is controlled with a heating rate of 5° C.
According to individual preferred practical examples the process occurs in a horizontal kiln through which the product being carburized passes.
The entire process is controlled by precise monitoring of the weight change of the product being carburized in its partial processes. Following the reduction step, the W powder is essentially completely converted to W
2
C (see Example 1 of the 797 patent) before WC is formed in another partial process.
According to Example 2 of the '797 patent the tungsten carbide end product has a total carbon content of 6.13 wt % and the BET surface of the powder was determined at 4.0 m
2
/g. A shortcoming in the mentioned process is that corresponding experiments were only conducted in a thermobalance unit or in an exceptional case in a mini-laboratory furnace with reaction amounts of up to 20 g. For lack of sufficient control, the critical process parameters of the '797 patent cannot be scaled up to commercial production. The very complex process control prescribed there, especially the exact temperature control and correlation with H
2
O partial pressure in the region of the product being carburized, cannot be economically transferred to equipment for industrial tungsten carbide production. For this reason, the method disclosed in the '797 patent is not technically feasible in ordinary charges of the product being carburized in ordinary industrial units.
The WC powders produced according to the '797 patent always contain undesired W
2
C powder fractions detectable in XRD recordings and generally undesired free carbon.
The '040 patent describes a carbothermic reaction process for production of nanophase metal/metal carbide particles, for example, WC—Co powder. The overall process is divided into three sections, a porous precursor particle being produced after step 1, which acts as substrate for penetration of carbon. In a second step carbon from a gas serving as the carbon source is allowed to penetrate the porous precursor particles at a carbon activity a
c
≧1.0 and in a concluding step carbon and the gas serving as the carbon source are simultaneously made to react with the precursor powder particles so that at least one carbide phase is formed and the remaining unconverted carbon is eliminated by gasification with a second gas that can combine with carbon.
The porous precursor particles are produced according to the description in a demanding wet reaction from known tungsten oxide precursors, like H
2
WO
4
or ammonium paratungstate by chemical dissolution and spray drying. Co(en)
3
WO
4
or also AMT-Co—Cl
2
are such precursor materials. (en stands for ethylenediamine and AMT is an abbreviation for the costly ammonium metatungstate.)
The carburizing reaction preferably takes place in a fluidized bed. 700 to 850° C. is stated as process temperature. CO gas is used as process gas for the WC reaction, which automatically means a carbon activity a
c
>1. To eliminate formed, unreacted free carbon after the actual WC carburizing process from the reaction zone, the process atmosphere is changed to a mixture of CO and CO
2
or CO/H
2
, as well as CH
4
H
2
with a carbon activity o
Filzwieser Andreas
Lackner Andreas
Hendrickson Stuart L.
Morgan & Finnegan L.L.P.
Plansee Tizit Aktiengesellschaft
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