Electric heating – Metal heating – By arc
Reexamination Certificate
2000-02-16
2001-03-27
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121510, C219S121470, C315S111510
Reexamination Certificate
active
06207924
ABSTRACT:
The present invention relates to inductive plasma torches and more specifically the use of such plasma torches in the making of products and materials resulting from the injection of reactives into the plasma at high temperature.
Such a system is for example described in French patent 2,714,371 (Cabloptic SA). This patent more specifically aims at the forming of silica on a pellet. In this patent, the plasma torch includes an outer tube, an intermediary tube and a central injector. Between the outer tube and the intermediary tube is injected a plasma gas, for example oxygen. A neutral gas, for example argon, is provided between the intermediary tube and the injector. The injector includes a central tube in which are injected the reactive products, for example, SiCl
4
and O
2
, and the central injector tube is surrounded by another tube. Between this other tube and the central tube is injected a sheathing gas, for example also argon. The exact function of the sheathing gas is not specified. It may be thought that it is intended for avoiding new depositions of the products formed from the reactive gases on the ends of the injector tube. The injector is clearly recessed with respect to the plasma formation area.
The present invention more specifically aims at the use of a plasma torch for the manufacturing of superfine powders, for example of tungsten or another refractory material, or alumina. To produce superfine powders from metallurgic powders, it is necessary that the injected products transit for a sufficient time through the core or central area of the plasma. This raises a difficulty since, due to electromagnetic forces (Laplace forces) acting on the plasma that is electrically conductive, the gas flow in the discharge is directed downstream (to the outlet) at the periphery of the plasma and is directed upstream in the axial area. Such recirculations tend to send towards the periphery the substances injected into the plasma. To force the reactives to pass into the axial area of the plasma, the speed of the injection gases is increased. But the substances that are on the edge of the injection flow are however driven by the recirculations towards the plasma periphery. This results in an inequality in the processing of the injected powders. Thus, with existing powder melting devices (spheroidizing, densification . . . ), irregularly processed powders are obtained, some being small and suitably spherical, others remaining sharp-angled, which makes them improper to certain uses. For example, in a method where metallurgic tungsten powders having a diameter on the order of 50 &mgr;m are introduced, a yield of 50 to 70% only of spheroidized (and thus melted) or superfine (and thus resulting from a total evaporation) tungsten powders appears to be obtained with existing methods while the rest substantially resembles the introduced powders.
Thus, the present invention aims at a plasma torch structure and at applications thereof improving the efficiency of conversion of a metallurgic powder into a small-grained powder or producing a small-grained powder from gaseous precursors.
To achieve these objects, the present invention provides an inductive plasma torch including an outer tube, an intermediary tube and a central injector including at least one central reactive injection tube and one peripheral sheathing tube. A gas of same nature as the plasma gas is injected into the space included between the central tube and the sheathing tube in conditions adapted to obtaining a laminar flow, this flow remaining laminar until the central plasma area of the torch is reached. The sheathing tube substantially emerges at the level of the first turn of the induction coil.
The present invention also aims at an application of the plasma torch hereabove to the forming of superfine powders of tungsten or any other refractory material, consisting of injecting into the central injection tube a metallurgic powder of tungsten or any other refractory material with a carrier gas such as a mixture of argon and nitrogen, the other gas injection rings receiving argon.
The end of the central tube is preferably slightly recessed with respect to the end of the sheathing tube.
The present invention also aims at an application of the plasma torch hereabove to the manufacturing of a superfine alumina powder, consisting of injecting an aluminum powder with a carrier gas such as a mixture of argon and hydrogen into the central injection tube, and oxygen into a peripheral tube, the other gas injection rings receiving argon.
REFERENCES:
patent: 4665296 (1987-05-01), Iwata et al.
patent: 5012065 (1991-04-01), Rayson et al.
patent: 5285046 (1994-02-01), Hansz
patent: 5907566 (1999-06-01), Seltzer
patent: 5908566 (1999-06-01), Seltzer
patent: 60012668 (1986-08-01), None
patent: 62274428 (1989-05-01), None
P.W.J.M. Boumans and F.J. De Boer, Studies of flame and plasma torch emission for simultaneous multi-element analysis-I Preliminary investigations, Spectrochinica Acta., vol. 27B pp. 391-414.
Buchanan & Ingersoll PC
Centre National de la Recherche Scientifique
Paschall Mark
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