GB2040650A - Channel induction furnace - Google Patents

Abstract

A channel of a channel induction furnace is characterised in that it is lined by a mixture consisting almost entirely of SiO2 with a little pure graphite and a little Na2B4O7, the mixture being compacted whilst in a dry state.

Description

SPECIFICATION Process for preparing, changing putting into operation and for increasing the electrical power of inductors of an induction furnace to be operated with line frequency and in a channel system, having been provided with changeable inductors, the inductive melting furnace being suitable for the production of OFHC copper and of copper desoxydated with phosphor, as well as two and multicomponent brass alloys.

The invention relates to a process for preparing, changing, putting into operation and for increasing the electrical power of the inductors of an induction furnace to be operated with linefrequency and in a channel-system, having been provided with changeable inductors, the inductive melting furnace being suitable for the production of OFHC copper and of copper desoxidated with phosphor, as well as two- and multicomponent brass alloys.

Equipment serving for copper melting operated at the presently known technicai level and having been internationally accepted are ready to receive 20 to 50 tons of molten metal. The melting furnaces are generally operated with line frequency in a channel-system and are provided with changeable inductors, where heating of the molten copper is performed in an inductive manner, in the channel-system arranged in the socalled inductor, forming a secondary part of the transformer. In the middle of the channel system there is the installed iron core together with the belonging insulated multicoil primary winding.

The main parts of the melting furnaces being suitable for copper production are formed by the changeable inductor units having been arranged on the mantle of the melting furnace, the number thereof varying between 2 and 6 at the single equipments.

In general when preparing inductors, steel stamping forms have been used, which are removed later from the channel-system. Mostly, the lower part of the channel is formed with a rectangular, the ascending part with a circular cross-section.

The stamping material mostly used for the preparation of inductor channels is composed of 50 to 55% Al203,40 to 45% SiO2 and O to 5% TiO2 + Zr O. The mostly spread and internationally accepted type is known under the name of "Tasyl 217 Dry." Literature disclosing the prior art is to be found in following articles: 1./ASEA Journal 1968. Volume 41, Number 5, Bernt Olausson, Furnace Department: "ASEA quick-change inductors for channel furnaces" 2.1 Giesserei 56/1969/Nr. 9. 25. April, Von Fritz iten, VDG, in Bülach und Max Reithmann in Fislisbach: "Der Rinnenofen im modernen Giessereibetrieb." 3./ Giesserei 57/1970/, Nr.9. 23.April, Frerking, W: "Feuerfeste Auskleidung und Betrieb von Induktionsinöfen in Eisengiesserein".

The stamping material mentioned is mixed with 4,2 to 4,5% H20 and after an aging process lasting for 24 hours, the stamping of the inductor may be begun. In course of said stamping process the stamping material is spread out in a thickness of about 60 mm and stamped down to a thickness of about 30 mm. Only after having stabbed the stamped part, the following layer may be applied.

After finishing the stamping of the inductor, the channel forms are removed from the inductor casing and after a 6-day airdrying to be performed at operational temperature, the inductor is to be placed into the dryroom. After a 3-day airdrying in the dryroom the resistance strips are led into the channels and connected to the multi-stage transformer; increasing the voltage of said transformer and taking a heating timetable of 30C/day into consideration, the temperature of the stamped inductor and channel-system should be increased to 160cho. The inductor is kept at this thermal value for 6 days, after this period the temperature is gradually reduced to 60cho and kept on this level till the day previous to installing onto the furnace-body.

By using the process described above, the inductor gets into an operable condition only after an utmost slow and wearisome heating process.

Careiessly performed stamping or rapid heating, oscillations of impacts occuring in course of assembly, all may cause the failure of the inductor, not mentioning the fact, that after the installation onto the furnace P'glueing on"/, even in case of an accelerated heating the inductor can be filled up with the liquid metal and put into operation only after a heating period of 96 hours.

The influence of the cracks occuring during drying or heating and caused by stresses appear at the electrical loading of the inductor; under the influence of power increase the cracks are filled up with the liquid metal, due to the strong current in the channel the metal is allowed to advance to the primary coil, making further loading and operation of the inductor impossible.

In order to eliminate the above problems, in accordance to the invention in contrast to the wet stamping process a pure milling product, containing minimum 99,5% SiO2 is used, which - after having been dried at 4000C 8 hours long - is mixed with 1,0 to 1,6 Ri Na2B407 sintering material. Max. 50% of the sintering material may be substituted by H3BO3. Stamping is performed in a dry state and in such a manner that into both of the lowest and upmost branches a tubular channel-form made of copper and ensuring the most ideal efficiency and exploitation of the induction is stamped into the inductor casing.

According to the invention in course of the preparation of the stamping material 2 to 4% pure graphite ground to flour fineness are added equally distributed to the fractioned milled product containing min. 99.5%SiO2. In course of the sintering of the channel wall the temperature of the liquid metal streaming in the inductor channel /1400 to 1 5000 Cl makes it possible, that the C particles introduced may form SiC compounds, thus considerably increasing the heat-resistance of the channels and prolonging the useful life thereof.

The inductor thus prepared may be immediately mounted onto the furnace. After having heated the tubular channel-forms for 8 hours by means of a gas-burner, the inductor may be filled up with the liquid copper/O2-contents < 100ppm/ and after the sintering process of the duration of 48 hours the applying of the electrical loading onto the inductor may be begun. This process does not disturb at all the operation of the dther inductors of the melting furnace; after having been filled up with metal, the melting furnace may be put into operation at full loading of the other inductors.

According to the invention it possible that by omitting the plugging places considered as sources of faults up to now, channels with a rectangular cross-section formed of copper tubes, being ideal from the point of view of electricity, are applied, by the use of which the electrical loadability of the inductor, the current density of the channel and thus the power loading may be considerably increased.

It should be also mentioned, that by forming the new cross-section of the channel, the thickness of the stamped fire-resistant layer arranged outside the channel can be increased without the necessity to the change the middline of the channel-system compared to the previous system. The obtainable increase in electric power is about by 10-15% higher related to the power of known inductors.

In order to be able to eliminate damages occuring in course of the assembly of the inductor, as well as to obtain the proper mechanical strength of the stamping material resisting to the high pressure of the metal and to the intense stream of metal in the channel, after having finished the stamping process, a part of the stamped layer amounting to about 50 mm is to be removed, while said stamping material having been mixed with 8% phosphoric acid is used for closing the upper part of the inductor. The chemical bond to be established in the closing layer should be promoted by producing a stream with a small gas-flame.

The change of an inductor and filling up the new inductor with metal can take place within 24 hours and not being compelled to change all the inductors arranged in one line on the melting furnace, the single faulty inductor can be individually changed.

Holding of the inductor lying on the furnace next to the place of change and turned out of the metal level can be easily performed by means of a gas burner without damaging the inductor not to be changed. By this method the number of changes of the inductors and economical exploitation thereof can be further increased.

Normal mode of operation according to the process of our invention will be described in details by the way of an example: The insert forms for closing the apertures of the channel-forms and of the iron core are made of copper. On one or two spots of the insert-forms there are breakers arranged made of a plastic in order to avoid heating up of said insert-forms when placed into the inductive tube space.

The fractioned stamping material containing min. 99.5% SiO2 is subjected to drying at 800 OC for four hours. When determining the quantity, of the stamping material, the quantity prescribed for the inductor casing, to be stamped therein should be also taken into consideration.

After having finished drying, the stamping material is mixed with 1.4% Na2B407, and with a pulverized sintering material, as well as with 3% graphite ground to flour fineness in an edge runner. When the insulation of the steel inductor casing with asbestos has been finished, and the wider side has been lined with one row of 20 mm thick refractory bricks, stamping may be begun.

Stamping should be performed continuously in dry state by means of a tubular resp. flat manual stamping tool with an edgelike shape. The single layers are continuously spread out in a thickness of about 30 mm, followed by the stamping process. After the layer spread out has been stamped, the inductor casing is knocked several times by means of a rubber hammer in order to promote compacting of the grains. After having stamped out the complete height of the inductor casing, the upper layer in a thickness of about 50 mm should be removed.

Onto the remaining stamped part ofthe inductor casing phosphoric acid is allowed to drip by means of a brush. The SiO2 stamping material previously wetted with 1% water and mixed with 8% phosphoric acid is spread onto the layer prepared in the described manner. In order to meet the requirements relating to the desired strength of the closing part, this wet part is rammed into the upper part of the casing by means of a jackhammer.

The assembly of the inductor with the coil and the iron core is performed. In order to promote chemical binding on the stamped surface, it is to be heated slightly on the stamped surface two or three times by means of a mild gas flame. In such a manner the inductor is ready for the assembly into the furnace. After finishing installation works, we may begin heating up the copper channelforms in such a manner, that the ventilators cooling the coils should be in operation with the purpose to avoid the deformation of the slit insert made of copper, arranged at the primary coil.

After-the heating of the channel system during 8 hours the inductor will be rotated under the level of the liquid metal and at a voltage of 130 to 1 80 V of the secondary coil of the transformer which is corresponding to a minimal channel load -the enameling and the sintering of the channels of the new inductor takes place. During said time the furnace can be filled up by metal and the equipment can be operated with the normal loading of the 'inductors being not changed.

After the expiration of the sintering period, under the steady control of the values of the thermo-elements built in into the stamped layers, gradual increasing of the capacity may be begun and within 6 days the inductor may be loaded with the maximal power.

Claims (5)

1. A process for preparing, changing, putting into operation and for increasing the electrical power of the inductors of an induction fumace to be operated with line frequency and in a channelsystem, having been provided with changeable inductors, the inductive melting furnace being suitable for the production of OFHC copper and of copper desoxydated with phosphor, as well as of two- and multicomponent brass alloys characterized in that the previously dried, fractioned stamping material containing min.

99.5% SiOz is mixed with 1 to 1,6% Na2B40, sintering material, as well as with 2 to 4% pure graphite pulverized to flour fineness and the mixture thus obtained is stamped in a dry state into the inductor casing, into which a copper channel-form ensuring the most ideal shaping from the point of view of electricity has been placed, while 50% of the sintering material metioned above may be also substituted by H3 BO3.

2. A process as claimed in claim 1, characterized in that the plugging apertures previously needed for heating up in course of the wet stamping process are omitted and by dimensioning the channel, in the previously described manner the electrical power of the inductor can be increased by 10--15%.

3. Process as claimed in claim 1 or 2, characterized in that in order to conterbalance the intense stream of metal in the channel at closing the inductor casing, the upper part of the inductor is stamped with a mixture containing quartzite and 8% phosphoric acid, furthermore in order to promote chemical bond, heating performed by means of light gas flame is used.

4. Process as claimed in any of the claims 1 to 3, characterized in that pure graphite pulverized to flour fineness, admixed in a quantity of 2 to 4%, ensures in course of sintering the channel the forming of SiC in the stamped layer being in a direct contact with the metal, thus increasing the heat resistance and the useful life of the channel.

5. Process, as claimed in any of the claims 1 to 4, characterized in that heating of the newly changed inductor and filling up the same with the liquid metal can be performed utmost quickly, within 24 hours, whereby the change and putting into operation of only one single inductor within the furnace becomes possible.