FI64876B - SAFETY CONNECTION FOR HETTING AV ETT MATERIAL MED HJAELPAV ETT STROEMPLASMA - Google Patents

Description

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Method and apparatus for heating materials by means of flow plasma

The invention relates to a method of heating various materials in a reaction vessel by means of plasma, in which case direct current and low voltage are used to generate the plasma, and to an apparatus for carrying out the method.

The heat required by pyrometallurgical processes can be introduced into the reaction vessel either directly inside the reaction vessel or through its jacket. The disadvantage of heat transfer through the jacket of the reaction vessel, especially in endothermic reactions, is that the temperature of the inner surface of the reaction vessel clearly exceeds the temperature of the material to be processed. This easily results in sintering of the material into the jacket of the reaction vessel and thus in a decrease in heat transfer. The end result may be sintering of the entire material mattress. The heat required by the process can also be obtained by performing heating inside the reaction vessel. A conventional way to accomplish this is to pass hot fuel gases through the reaction vessel. However, the result is a large amount of gas to be treated and a large amount of airborne dust. Flue gases can also participate in the reactions.

One practical solution for heating inside the reaction vessel is to use an arc. In arc heating, the electrodes wear and, as they wear, they have a reducing effect on the furnace atmosphere.

The device of U.S. Patent 4,006,284 uses an arc between the electrodes to heat the material so that the material is fed through the arc. For this reason, a high voltage must be used so that the electrodes can be moved far enough apart. Em. the device uses alternating current and Ar, He, CO or meta-64876 is fed between the electrodes. Feeding these is said to have an arc stabilizing effect.

Plasma formed from the ionizable gas between the electrodes has been used to heat the reaction vessel. The heating used in the known methods can be indirect: the plasma is formed, for example, in a quartz tube, past which flows a gas which is used as a heat transfer to the reaction vessel. Disadvantages of the method are the effect of a large amount of carrier gas on the process and the poor heat transfer capacity of the gases.

The use of plasma to heat the materials inside the reaction vessel is more advantageous for many purposes than other alternatives: large amounts of carrier gas are not required to bring heat to the reaction vessel as in indirect plasma heating, and the plasma does not affect the furnace atmosphere as much as using an arc. The heating methods used in rotating reaction vessels are either heating the jacket of the reaction vessel or introducing fuel gases into the reaction vessel.

When forming plasma, the number of electrodes may vary. In some cases, there may be only one electrode; then a plasma is formed between the electrode and the furnace charge.

There may also be several electrodes; the most common number is probably two, with one electrode being anodic and the other cathodic. The most common material for electrodes is carbon.

The ionizable gas can be introduced between the electrodes in many ways. The gas can be introduced as a separate jet to the ionization site and can be introduced around the electrode. One preferred way has proven to be to introduce gas through a hole drilled in the electrode.

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The electric current used to generate the plasma can be either direct current or alternating current. The alternating current used can be either single-phase or three-phase.

Plasma heating power is usually controlled by adjusting current and / or voltage using a transformer.

In accordance with the presently developed invention for heating materials, plasma is generated in a reaction vessel using direct current and low voltage. When the plasma is generated in a rotating reaction vessel, the electrodes are shaped so that the material to be heated does not get between the electrodes as it rotates. In addition, the heating power of the plasma can be adjusted by adjusting the flow of ionizable gas. When plasma is generated in a vertical furnace, only one electrode is used in the furnace, the bottom of the furnace acting as another electrode.

It has now surprisingly been found that the current can be increased by increasing the amount of ionizable gas while the voltage remains at the desired level of 18-24 V. All the direct current power goes to generating the plasma and through it as radiant heat to the furnace. The plasma temperature depends on the gas used and its amount, as well as the power supply. As the temperature of the electrodes becomes high, the ionization of the plasma gas is facilitated while its volume increases. For this reason, the gas flow or power used can be reduced to achieve constant heat transfer. According to the method now developed, the temperature of the furnace can be adjusted by adjusting the plasma gas valve according to a pre-programmed schedule.

Plasma is generated from an ionizable gas such as argon between two DC-controlled carbon electrodes by a tesla discharge. The plasma thus formed is called current plasma. A cathodic voltage is connected to the second and an anodic voltage to a second electrode with a hole in the center for conducting the ionizable gas. The thickness of the carbon electrodes depends on the power used.

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The carbons acting as electrodes in this case are only electrically conductive elements, they do not wear like the electrodes forming the arc, but the wear of these electrodes is only a small amount of evaporation.

The effect of direct current results in a very stable plasma. Due to the low voltage, only readily available gas such as argon can be used in the plasma space. Because a low voltage of 18 to 24 V is used, the equipment can be made simple, as there are no breakthrough problems or safety risks.

The electrodes are protected against combustion by nitrogen or some other shielding gas. Initial ionization is generated between the electrodes by a tesla discharge. The distance between the electrodes is then adjusted to suit and a readily ionizable gas such as argon is passed between the electrodes to provide the desired ionization and electric current.

In the following, the invention and the advantages obtained by it will be explained in detail with reference to the accompanying drawings, in which Figure 1 shows a cross-section of a preferred embodiment of the invention, a rotary tube furnace.

Figure 2 shows a cross-section of electrodes for tube furnace use.

Figure 3 shows a cross-section of another embodiment of the invention, a vertical furnace.

The apparatus according to Figure 1 comprises a rotary tube furnace 1 supplied from a DC power supply 2, the AC supply of which is marked 16. The apparatus also includes voltage-current stabilizers 3 and voltage-to-current setpoint adjusters 4. The temperature measurement in the furnace 5 64876 takes place at point 5 ( in practice, the measurement takes place at several points, but for simplicity only one point has been taken into this). The temperature control in the device 6 takes place following the temperature program 7 prepared for each process, so that depending on the program prepared and the oven temperature, the control system 6 opens and closes the valve 8, through which the ionizable gas 17 is introduced into the oven. The anodic electrode 9 and the cathodic electrode 10 are led from the furnace ends to the furnace space. The ionizable gas flowing through the anodic electrode 9 is ignited by means of a tesla discharge, and this ignition takes place by means of the ignition unit 11. The electrodes have water cooling 12 and a hole is drilled in the anodic electrode guide holder (not shown in detail), through which the shielding gas 13 is introduced into the furnace space. From the furnace, the gases exit through a hole 14 drilled in the cathode electrode holder (not shown). The oven is filled and emptied through the opening 15.

Figure 2 illustrates the ends of the carbon electrodes formed to form plasma in a rotating reaction vessel. The end 18 of the cathodic electrode 10 is spherical and the end 19 of the anodic electrode 9 is concave, cup-shaped corresponding to the shape of the cathodic carbon (electrode). The distance between the electrodes is adjusted between 1 and 5 mm. The dashed arrows between the electrodes have been used to illustrate the outflow of plasma gas.

Figure 3 shows a vertical furnace 1 with a powdery material 25 inside, e.g. for melting. As in Fig. 1, reference numerals 2 to 8 describe power supply and control equipment and a temperature control system. The anodic electrode 9 extends from above close to the bottom of the furnace. The cathodic electrode 10 acts as a graphite base for the furnace, and when molten material has accumulated on the bottom of the furnace, this melt acts as a cathodic electrode. In this case too, the ionizable gas is ignited by means of the ignition unit 11. The anodic electrode 9 is supported by a support arm 20, by means of which it is also connected to the electrode transfer device 21. By means of the transfer device, the electrode can be raised and lowered. The second part of the transfer device is formed by a motor M. There is an insulator 22 between the support arm 20 and the electrode. Both electrodes are water-cooled. The cathodic electrode 10, i.e. the graphite base, shrinks outwardly from the furnace into a rod-like electrode surrounded by a water-cooled electrode holder 23. The cooling water valve which appears here is marked with the number 24.

In a rotary tube furnace, the entry of heated material into the plasma space is prevented in many ways. First, the electrode ends are shaped (spherical - concave) for this purpose. The pressure exerted by the plasma gas also prevents the material from entering the plasma space. It is also possible to use a shielding gas to protect the carbon electrodes.

In the vertical furnace, only one electrode, i.e. the anode 9, is embedded in the furnace space, the graphite bottom of the furnace acting as a second, cathodic electrode. Again, the ionizable gas is passed through the anodic electrode between the electrodes, and the plasma is generated at the bottom of the furnace. As the temperature rises, the material to be melted melts, and also acts as a cathodic electrode at the bottom of the furnace. Depending on the electrical conductivity of the melt and the surface height of the melt, the anodic electrode can be raised. Electrode lifting can be automatically connected to power consumption. The melting material of the furnace is filled from above. At the top of the furnace, at lower temperatures, the mattress material to be melted may be somewhat electrically conductive, but its conductivity decreases as the temperature rises, until again, as a metal melt, it is electrically conductive. The electrical conductivity of the carbon electrode 9 in the furnace space increases again as the temperature rises. When the electric current in the furnace flows from the cathodic to the anodic electrode, i.e. from the bottom up and by the descending mattress material of the electrically conductive conductor downwards, all the electric current passes only through the electrode. At the same time, the adhesion of the mattress material to the electrode is avoided. The vertical furnace is well suited for the analysis of small amounts of elements, for example.

The method of heating the material according to the invention is very advantageous when it is desired to study the exhaust gases generated in the reaction vessel. When the current plasma is generated in the reaction vessel, the gases leaving it are either shielding or plasma gases or exhaust gases. The quantity and quality of the shielding and plasma gas are known and it is easy to calculate the quantity and composition of the exhaust gas from this.

Example

A rotary kiln with adjustable speed was used as the test equipment. The electrodes were inserted from the round openings at the ends of the oven into the oven. Two 16 kW rectifiers were used as the power supply, the voltage of which could be adjusted to 0-40 V. The current supplied to the furnace was 800 A and the voltage was 20 V. The diameter of the cathodic carbon electrode was 60 mm and that of the anodic 90 mm. In addition, a hole was drilled in the middle of the anodic electrode, through which argon gas was introduced. The equipment has been in trial use for six months and has met the requirements set for it.

Claims (9)

1. A method of heating a material in a furnace by forming an ionizing gas between a current plasma between electrodes in the material to be heated, characterized in that the heating effect of the plasma is controlled by controlling the flow rate of the plasma gas. 2. A method according to claim 1, characterized by the use of direct current. 3. A method according to claim 2, characterized in that the voltage of the direct current is about 18-24 V. 4. A method according to claim 1, characterized in that the plasma gas is measured between the electrodes by a longitudinal shaft of the anodic electrode and that the flow rate of the gas is controlled by regulating a flow valve according to a predetermined automatic program. 5. A method according to claim 1, characterized in that the electrodes are additionally protected by conducting protective gas, such as nitrogen into the electrodes. 6. Apparatus for heating a material in a furnace by means of an ionizing gas forming a current plasma between electrodes in the material to be heated, characterized in that at least one electrode (9, 10), advantageously the anodic (9) ) is hollow, wherein the plasma gas is intended to be passed between the electrodes via the hollow electrode in an amount controlled by a current supply and control device (2-5) and temperature control device (6, 7). 7. Device according to claim 6, characterized in that the end of one electrode (eg 10) is cup-shaped concave and the other (9) corresponding convex.