US3461339A - Electric arc stabilization in electric arc melting using carbon electrodes - Google Patents

Description

US. Cl. 313-657 8 Claims ABSTRACT OF THE DISCLOSURE An arc melting method for use in the melting of metal and in the production of metal from metal-containing ore and a reducing agent, in an arc furnace. The method stabilizes an are formed between the metal in the furnace and at least one carbon electrode by providing the electrode with a core including a substance which, under the action of the arc, ionises to increase the electrical conductivity of the arc. The substance included in the core of the carbon electrode may be lithium, barium, calcium, magnesium, chromium, manganese, silicon, potassium, sodium, rubidium, or caesium or compounds such as oxides, carbonates, or silicates of such elements. Such substance is added in particular form and bonded with a binder such as tar, pitch, or a paste or anthracite, tar and pitch.

This invention relates to the stabilisation of electric arcs and to the provision of electric arc apparatus, and more particularly electric arc furnace apparatus capable of conferring improved stability to the arc.

In larger direct arc melting furnaces, by which is meant furnaces in which the arc is struck between a carbon electrode and the metal to be melted, one problem is the heavy surging which occurs when making the are since a virtual short circuiting condition is present. The extent of the surging is kept within limits by having a reactance in the circuit, but where larger power inputs are involved, such as ten to forty thousand kva. the surging is sufiicient to cause flicker on the supply system.

It is an object of the present invention to reduce such surging effects and to obtain other advantages such as improved power efliciency and lower electrode consumption.

According to the present invention there is provided a method for the stabilisation of an electric are which comprises introducing into the arc a substance which under the action of the arc generates, or is converted into, a vapour or median which increases the electrical conducitvity of the arc.

The introduction of the said substance may be eiiected by any of a number of different methods. Thus the substance may be included in or on one or more arc electrodes or the substance may be introduced into the are as gasborne particles, e.g. by spraying the substance into the arc, e.g. through a hollow electrode or introducing it via a lance, 'or the substance may be provided as a solid rod which is continuously fed into the arc at a rate balancing its rate of consumption in the arc. Or a plurality of the foregoing methods may be used in combination in order to ensure a sufficient supply of the substance to create ionised particles thereof in the arc and thus to increase the conductivity in the path of the arc.

The invention further includes electric arc apparatus and more particularly electric arc furnace apparatus wherein at least one of the arc electrodes carries or embodies a substance as aforesaid and/ or wherein means are ice provided to introduce a said substance into the are from an external source.

The invention will be described with particular reference to the case where the said substance is included in, or carried on, at least one of the arc electrodes.

The invention is of particular value where the arc electrode or electrodes are formed essentially of carbon (e.g. graphite).

In selecting the substance for inclusion in the electrode or electrodes regard should be had to the purpose to which the arc is being put, so that materials deleterious to that purpose are avoided. Specifically it is desirable to select a material of relatively low cost, which is readily worked (and, if a metal compound, is readily reduced) and which provides adequate vapour or other medium of suflicient conductivity. Where an electric arc furnace apparatus used for melting metal is concerned, the selected substance should be one which has no contaminant or other deleterious effect on the metal to be melted or on the furnace structure. Within these desiderata a wide range of substances is available for use. Generally it is convenient to use metals per se and these may be included from relatively low melting metals e.g. lithium, metals of medium melting point e.g. barium, calcium and magnesium, and metals of high melting point, e.g. chromium and manganese. Certain nonmetals, e.g. silicon, may also be employed.

Instead of using the metal per se a compound of the metal, or for example of silicon, may be employed with reliance on other ingredients present in the electrode to cause reduction of the said compound. Thus metal oxides, carbonates and silicates are very useful. Where, as will be most common the electrode or electrodes are formed essentially of carbon this may itself serve as the reducing agent. However other reducing agents may be employed, e.g. for the reduction of manganese dioxide, silicon or aluminium may be included.

It will be understood that several substances which afford the conductive vapour or medium may be used in admixture. A mixture of substances of particular value is provided by burnt dolomite which comprises calcium and magnesium oxides.

It has been found that the inclusion of compounds of the alkali metals, namely lithium, sodium, potassium, rubidium and caesium, is particularly effective in lengthening the arc and improving its stability. It is thought that this is due to their low ionisation potential, i.e. their readiness to ionise in the are thereby increasing the electrical conductivity.

On the other hand, compounds of sodium and lithium in particular are very readily vapor-ised and their use is less preferred since their presence in any substantial concentration in a furnace tends to flux the refractory materials of which the furnace lining is made. Potassium is not so detrimental in this respect and its compounds have been used very successfully for incorporation in graphite electrodes, as will be demonstrated in the examples which follow. Rubidium and caesium are effective but their use is difiicult to justify in view of their high cost.

The substance relied on for the provision of the conductive vapour may be included in the electrode or electrodes in any convenient way. Thus it may be included as an ingredient of the composition of which the electrode is made or it may be provided as a sleeve or core to the electrode. Generally it will be present in particulate form and bonded together with a binder which may be for example, tar or pitch, or a paste made from anthracite, tar and pitch.

Particular advantages have been found to accrue from the use of a mixture of oxide in an excess of carbon paste, e. g. a paste of anthracite, tar and pitch. When this is baked to form the electrode the product is found to have a cellular or porous structure. When the electrode thus formed is put to use and so becomes heated the dispersed oxide is reduced to metal which is retained in the pores of the electrode until, by the heat generated in the are, it is vapourised. The availability of the metal for the purpose of the present invention is thus enhanced. An analogous result is obtained if the oxide is dispersed in a coking coal.

For most purposes the substance of choice has been found to be a potassium salt, e.g. potassium carbonate alone or with other substances, in a carbon electrode.

The optimum selection of materials and mode of introduction thereof is determined largely by the use to which the arc is to be put. For example:

(a) When melting zinc in indirect arc furnaces, or zinc containing alloys, zinc oxide or zinc drosses are preferably used.

(b) When melting steel in large arc furnaces it is preferred to use metals which will not substantially affect the quality of the resultant steel, preferably silicon and manganese, or oxides thereof. If the metals themselves are used they are preferably in the form of ferrosilicon or ferromanganese. When the oxides are used, a paste of the oxides is preferably compounded as indicated above.

By the method of the invention not only is the surging referred to above reduced or eliminated but flicker and the creation of harmonics in the supply system are also reduced or eliminated.

The steadier are conditions enable more power to be developed in a given time and tend to improve the load factor. This higher load factor has the effect of increasing the melting rate and the output of the arc furnace. Moreover, the noise of the arc is considerably reduced, which is an important advantage, especially for indirect arc furnaces.

Further, the electrode control system need not be so sensitive or responsive and can, therefore, be of a more simple design.

The following examples will serve to illustrate the invention.

Example I A test was carried out on a 150 kva. single phase indirect arc furnace, i.e. a furnace in which the arc is struck between two carbon electrodes. The electrodes in this case were of graphite and were 4 inches in diameter. The electrodes were bored with a concentric hole 1% inch diameter which was packed with a core consisting of 60% anthracite, 20% pitch with a melting point of 50 C., 16% powdered glass and 4% potassium carbonate. These materials were premixed and rammed into the hole in the electrode which had previously been scored to provide a mechanical key for the core and was heated to 40-50 C.

The arc conditions were observed using standard graphite electrodes when the furnace was used for melting 750 lb. charges of phosphor-copper. Under these conditions the furnace load fluctuated between 105 and 145 kva., the fluctuations being most pronounced at the start of the melt when the charge was cold. The wave form of the electric supply was extremely irregular and the noise emitted by the arc was very considerable.

The test was repeated with the cored electrodes made up as described, but with the same phosphor-copper charge. A remarkable improvement Was immediately apparent; the supply wave form was practically sinusoidal, the load fluctuations were reduced within the limits of 140 to 150 kva. and the noise emitted by the arc was diminished to a very low level. The melting time was reduced by 15% and the energy consumed in melting the charge was reduced by over Example II The following compositions were made up:

(1) Ferromanganese (approximately 20 grams), 10%

sodium silicate (binder).

(2) Mn0 (approximately 15 grams) coke (approximately 5 grams) and 10% sodium silicate (binder).

(3) MgO (approximately 15 grams) coke (approximately 5 grams) and 10% sodium silicate (binder).

These mixtures were rammed into holes diameter by 1 /2 in graphite electrodes, which were then baked for 2 hours at 200 C.

A first experiment was carried out on two control electrodes. This was done by striking and lengthening the arc. Owing to the arcs dying out this procedure was repeated three times before sufficient heat was obtained in the furnace. Marked fluctuations in the electrical load ranging from 1530 kva. were observed. The maximum length of arc was 7 Following the introduction of the electrodes containing composition (1), a marked improvement was immediately observed. Considerable reduction in noise resulted and no fluctuations occurred, the furnace electrical load being steady at 25 kva. By this method the length of the arc was increased immediately to 1%-l /z and held for 2 minutes, during which period the evolution of brown fumes was observed.

On using the electrodes containing composition (2), the maximum length of arc obtainable in the test furnace, approximately 2", was immediately attained and held for 7 minutes, before switching the furnace off. Appreciable amounts of fumes were observed up to a period of approximately 5 minutes. Again considerable reduction in noise resulted and during the whole period the kva. reading remained steady at 25.

Electrodes containing composition (3) were then used. Again considerable improvement resulted, the kva meter being steady at 22 and a maximum arc length of 2 was obtained immediately. Copious white fumes were observed for a period of 7 minutes and the furnace left on for 15 minutes without any fluctuations occurring. After 15 minutes, the kva. reading had increased to 18.

While for the sake of succinctness the invention has been described only with relation to the method wherein the conductivity increasing substance is provided in or on an electrode, it will be understood that it may alternatively be supplied by any other of the methods referred to earlier herein, with analogous results.

I claim as my invention:

1. In the melting of metal and in the production of metal from metal-containing ore and a reducing agent, in an arc furnace, a method for the stabilization of an electric arc comprising forming the are between said metal and at least one electrode which is a carbon electrode having a core including a proportion 'of a substance which, under the action of the arc, ionises to increase the electrical conductivity of the arc, the said substance being selected from the group consisting of lithium, barium, calcium, magnesium, chromium, manganese and silicon.

2. In the melting of metal and in the production of metal from the metal-containing ore and a reducing agent, in an arc furnace, a method for the stabilization of an electric are which comprises forming the are between said metal and at least one electrode which is a canbon electrode having a core including a proportion of a substance which, under the action of the arc, ionises to increase the electrical conductivity of the arc, the said substance being selected from the group consisting of oxides, carbonates and silicates of lithium, barium, calcium, magnesium, chromium, manganese and silicon.

3. In the melting of metal and in the production of metal from metal-containing ore and a reducing agent, in an arc furnace, a method for the stabilization of an electric arc which comprises forming the are between said metal and at least one electrode which is a carbon electrode having a core including a proportion of a substance which, under the action of the arc, ionises to increase the electrical conductivity of the arc, the said substance being selected from the group consisting of compounds of lithium, sodium, potassium, rubidium, and caesium.

,4. A method as set forth in claim 3 wherein said substance is selected fnom the group consisting of oxides, carbonates, and silicates of lithium, sodium, potassium, rubidium, and caesium.

5. The method of claim 11 wherein the said substance is present in particulate form and the core is bonded With a binder selected from the group consisting of tar, pitch and a paste of anthracite, tar and pitch.

6. The method of claim 2 wherein the said substance is present in particulate form and the core is bonded with a binder selected from the group consisting of tar, pitch and a paste of anthracite, tar and pitch.

7. The method of claim 3 wherein the said substance is present in particulate form and the core is bonded With binder selected from the group consisting of tar, pitch and a paste of anthracite, tar and pitch.

8. The method of claim 3 wherein the said compound is a carbonate.

References Cited UNITED STATES PATENTS Ladoff 3l3354 X Bassett 3l3354X Mott 313-354 X Dorcas 313354 X Parisot 313-654 Parisot 313-354 X Simon et a1 313354 X Parisot 313354 Wasserman et a1. 313-357 X Lauzau et a1. 313-357 X 15 JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner US. Cl. X.R.