GB2039761A

GB2039761A - Method and apparatus for stirring molten metal

Info

Publication number: GB2039761A
Application number: GB7901108A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-01-11
Filing date: 1979-01-11
Publication date: 1980-08-20
Also published as: GB2039761B

Abstract

For stirring molten metal in a furnace 6, a portion of metal is alternately withdrawn upwardly from beneath the surface of the body 5 of molten metal and expelled into a lower region thereof through a stirrer pipe 7 which is mounted for axial reciprocation and angular swinging to thereby enable the submerged jet of expelled metal to be projected in a fan-like fashion within the limits of an angle corresponding to the dimensions of the furnace 6, thus providing for effective stirring of the entire body 5 of molten metal. Stepwise reciprocation of the pipe 7 is effected by means of a pneumatic positioning device 19. The frequency of the metal-expelling cycles is varied during the operating process. <IMAGE>

Description

SPECIFICATION Method and apparatus for stirring molten metal The present invention relates to a method and apparatus for stirring molten metal in a container, e.g. directly in a furnace where the metal is melted, such stirring being advantageously effected to enhance the rate of melting or to maintain uniformity of composition or temperature in a standing body of molten metal.

A variety of methods have been used for stirring molten metal directly in the bath of a melting furnace, including mechanical, electromagnetic, gas dynamical and others. This invention proposes a method for stirring molten metals, such as aluminium alloys, which is both effective and simple to carry out.

This invention provides a method for stirring a body of molten metal in a metal melting operation, comprising alternately withdrawing a portion of molten metal upwardly from the body in a confined space to a level above the body and expelling the withdrawn portion of molten metal into the body under the action of compressed gas, wherein the said portion of metal is withdrawn directly from beneath the surface of molten metal and is expelled into a lower region of the body, adjacent to the bottom part of a metal holding means, the said alternate metal-withdrawing and metal-expelling steps being effected so that regardless of an increase in the level of the body of molten metal the portions thereof are always withdrawn from beneath the surface of molten metal and are expelled into a lower region of the body.

Such procedure permits enhanced stirring to be effected throughout the body of molten metal and permits the duration of a melting process to be reduced owing to favourable conditions enabling intensive heat and mass exchange which occurs in the body of molten metal due to the fact that molten metal is withdrawn directly from beneath the melt surface, having a substantially higher temperature, and is then discharged into a lower region of the body of molten metal, having a lower temperature. Such metal-withdrawing and metal-expelling procedure is especially effective at the initial stage of a melting process when a jet of overheated metal is poured over lumps of solid charge in a furnace to thereby enable thier rapid melting.

It is advantageous for each portion of molten metal to be discharged into the body of molten metal in a fanlike fashion as a jet of metal projected within the limits of an angle exceeding the angle of a free flow of the submerged metal jet and corresponding to the dimensions of the container, thereby enabling effective stirring of the entire melt. Such operation permits enhanced stirring of the entire body of molten metal, which is due to good stirring effect produced by the discharged jet of metal during each metalexpelling cycle on a substantially larger area of the melt. In addition, the jet of metal discharge in a fan-like fashion may be accompanied by turbulence created in this large area of the metal bath, which may be large enough to embrace the entire area of the metal bath bottom.

During a melting process, the expelled metal is preferably projected in a fan-like fashion at a progressively increasing angle one to twelve times the angle of free flow of metal jet per single pulse.

Such an operating procedure makes it possible to expand production potentialities of the melting process, as well as to enhance the effect of stirring, owing to the fact that, at the initial stage of melting solid charge, the jet of expelled metal is caused to follow an arcuate path within an angle embracing this lump of solid charge, i.e. within a confined zone. After the charge has been melted down, when the melt temperature is to be stabilized, the zone within which the metal jet is discharged should preferably be extended to cover the entire bottom area of the melt bath.This permits a substantial reduction in energy losses, since owing to the control of the stirring operation, effected by means of projecting the jet of expelled metal in a fan-like fashion at a variable angle in the body of molten metal, favourable conditions are created to enable enhanced heat and mass exchange in the body of molten metal.

The frequency of the metal-withdrawing and metal-expelling cycles is preferably decreased in a stepwise manner from 30 to 2 cycles per minute.

As a result, the quality of metal is improved by preventing the contamination of metal with oxide and slag inclusions during incorporation of alloying components, and by maintaining uniform temperature throughout the melt bath.

When there is still a small amount of liquid metal in a melting chamber and its temperature is very low, the frequency of the metal-withdrawing and metal-expelling cycles should be at its maximum in order to prevent the freezing of metal in the pipe cavity. A maximum frequency of the above-mentioned operating cycles, about 30 cycles per minute, is also necessitated by the fact that the traveling path covered by liquid metal is rather short, being, in addition, hindered by the lumps of solid charge.

When, however, the melt bath is to be maintained at a prescribed temperature prior to and in the process of discharging the metal from a melting furnace into a holding furnace or into any other vessel, the frequency of such operating cycles should be at minimum in order to prevent turbulence an undesirably high slagging of metal.

The consumption of compressed gas required for the stirring operation is also reduced in this case.

The method according to the invention for stirring molten metal is carried into effect by means of an apparatus which preferably comprises a gas pump with a pipe having its lower end submerged in the body of molten metal, the other end thereof being fitted with a nozzle brought in communication through a compressed gas supply means with a compressed gas holding means, and a vacuum system connected to said upper end of the pipe, wherein the said pipe is fitted in a refractory-lined cover above the metal holding means and mounted in guides for reciprocation along the axis thereof and connected with an actuator enabling stepwise reciprocation of the pipe in the metal body at a regulable speed and length of travel, as well as with an actuator intended for initial positioning of the pipe within, and its removal from the melt body.

Such apparatus construction permits effective stirring of the entire body of molten metal, avoiding the need of any substantial reconstruction of a melting furnace, owing to the fact that the stirrer pipe is mounted for reciprocation, preferably in a refractory-lined cover, above the metal bath in the guides. The actuator for stepwise reciprocation of the pipe is provided to ensure optimum operating conditions for the molten metal to be withdrawn directly from beneath the melt surface and to be expelled into a lower region of the melt. By regulating the speed of the pipe travel in the melt, it becomes possible to coordinate the stirring operation with the frequency of the metal-withdrawing and metalexpelling cycles.The provision of the actuator, for the initial positioning of the pipe and its removal from the melt, permits the prolonging of the service life of the pipe by virtue of sharply decreasing the pipe residence time in an aggressive medium.

Although a variety of different embodiments may be utilized as the actuators enabling stepwise reciprocation of the pipe, a very effective one comprises a pneumatic positioner provided with discharge outlets and with regulable throttle valves adapted to communicate the discharge outlets with the atmosphere. A positioning device of this type ensures effective operation of the pump regardless of the level of melt in the bath, since, as the level of the melt bath alternates, it becomes possible to change positions by communicating one of the discharge outlets with the atmosphere. A desired rapid operating rate is likewise ensured by appropriately adjusting a respective throttle valve in the discharge line.

It is advantageous that a ball-and-socket joint be positioned at the section of the pump pipe extending beyond the refractory lined cover of the container, as well as an actuator intended to enable angular displacement of the pump pipe about the said joint. As a result, it becomes possible to project the expelled metal in a fan-like fashion in the body of molten metal and to thereby achieve effective stirring of the latter throughout a substantial area, because the pump pipe is enabled, by means of the actuator for angular displacement, to perform turning motion during each metal-withdrawing and metal-expelling cycle.

From the above it follows that the invention is efficient enough to yield good results and superior advantages in metal-melting practice, permitting effective stirring of metal in a melting furnace with a minimum number of means and devices.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic vertical section showing the metal-withdrawing stage of a method of stirring a body of molten metal in a container; Figure 2 similarly shows a phase of the metalexpelling stage of the stirring method; Figure 3 is a schematic horizontal section showing the fanlike projection of the metal jet discharged into the body of molten metal; Figure 4 is a vertical section showing apparatus for stirring molten metal, and a container for the molten metal.

In Figure 1, showing the withdrawal of molten metal from beneath the melt surface, by means of a pump pipe: h1 is the height to which molten metal covers the open lower end of a pump pipe at the moment of the metal-withdrawing action.

In Figure 2, showing the delivery of molten metal from the pipe to the body of molten metal under the action of compressed gas: h2 is the height to which the open lower end of the pump pipe is covered with molten metal at the moment of its delivery: P is the pulse of compressed gas; Shown in Figure 3 is the fan-like projection of the expelled jet of metal, effected by means of tuming movement of the pump pipe, wherein a is the angle of free flow of the submerged jet of metal (for aluminium alloys this angle is about 150).

Figures 1 to 3 are simplified views showing the basic structure of one form of melting fumace 1 containing a body of molten metal, from which a metal portion 3 is withdrawn under suction from upper overheated layers of the body to the interior of a stirrer pipe 2 of a gas pump. Then pulse P of compressed gas is applied to the withdrawn metal in the pipe, which is forcefully expelled therefrom to come in contact with a lump of solid charge 4, thereby enabling intensive melting of the charge material.

The jet of metal expelled from the stirrer pipe is projected in a fan-like fashion, for which purpose the pipe 2 is caused to perform a turning motion within a sector whose angle is varied to be 1 to 1 2 times the angle of free flow of the submerged jet of metal, with a definite initial position of the pipe axis being taken as a point of reference.

It is possible to effect the stirring procedure in a manner described above, i.e. by varying the angle at which the metal is spread in the body of molten metal, from a control board or in accordance with a preset programme by means of a computer through the intermediary of a suitable actuator enabling the turning movement of the stirrer pipe.

The stirring procedure can be carried out either with or without stepwise alteration of the angle of the stirring zone.

The method will be further described by the following illustrative Examples.

EXAMPLE 1 An aluminium alloy was subject to melting under stirring effected by means of a gas pump throughout the melting process in a 30-ton melting furnace. The gas pump was operated at a variable compressed gas pulse frequency of 4 to 20 pulses per minute. The metal was drawn in from upper layers beneath the melt surface, the depth h1 of the stirrer pipe immersion in the melt being 100 mm. The withdrawn metal was then discharged into lower layers of the melt body, the depth h2 of immersion being varied from 300 to 600 mm as the level of the melt increased during the process of melting.When the withdrawn metal was expelled into the body of molten metal, the stirrer pipe was caused to turn within a sector whose angle was progressively varied from 45 to 1 200 (from 3 to spa). The stirring operation commenced when the temperature of the melt was 660 to 6700 C, with the stirrer pipe being immersed in the melt to a depth of 100 mm.

The compressed gas pulse frequency was about 20 pulses per minute in the course of first 10 minutes of the stirring operation, and the sector within which the metal jet was discharged constituted about 450 (about 3a). The molten metal then underwent stirring until complete dissolution of solid charge in the melt bath for a period of 20 minutes, the compressed gas pulse frequency being 1 5 pulses per minute, the metal jet discharging sector constituting about 900 (about 6cur).

Upon tapping off slag at the melt temperature of 680 to 6900 C, alloying components such as manganese and titanium were incorporated into the melt, and the stirring operation was then conducted at the pulse frequency of about 10 pulses per minute, with the metal jet discharging sector being increased to be about 1200 (about 8a). The dissolution of the alloying components was conducted for a period of 30 minutes, the melt temperature was raised and made uniform throughout the body of molten metal to be 730 to 7350C.

Thereafter, the stirring operation was performed with the compressed gas pulse frequency being 4 pulses per minute, the metal jet discharging sector being 1200. (about 8a).

This operating mode was maintained until the moment of pouring the molten metal into a holding furnace. The metal was found free from slag or any other impurities. The change in the mode of stirring was conducted from a control board.

With the above-described stirring procedure it became possible to enhance the effectiveness of stirring and to reduce the time of melting operation by 20 to 25 per cent.

As has been mentioned, the stirring procedure, during which the compressed gas pulse frequency is varied stepwise, may be run from a control board or effected in accordance with a preset programme by means of a computer.

Alternatively, the stirring operation can be conducted withoutstepwise alternation of the compressed gas pulse frequency.

EXAMPLE 11 An aluminium alloy was subject to melting under stirrring effected by means of a gas pump throughout the melting process in a 40-ton melting furnace. The gas pump was operated at a variable compressed gas pulse frequency of 2 to 30 pulses per minute (the gas used was nitrogen).

The stirring operation commenced after a bath of molten metal was formed to cover the lower end of the stirrer pipe, the melt bath temperature being 660 to 6700 C.

The compressed gas pulse frequency was about 1 5 pulses per minute in the course of first 10 min of the stirring operation. Then the molten metal underwent stirring for 25 min until complete dissolution of solid charge, with the compressed gas pulse frequency being 12 pulses per minute.

Upon tapping off slag, the melt temperature being 680 to 6900 C, alloying components such as manganese and titanium were incorporated into the melt, whereupon the stirring operation continued at a compressed gas pulse frequency of 8 to 9 pulses per minute. The dissolution of the alloying components went on for 30 min. The melt temperature was raised and made uniform throughout the body of molten metal to be 730 to 7350C.

Thereafter, uniform temperature of the melt was maintained in the course of stirring operation, the compressed gas pulse frequency being 4 pulses per minute at the moment of pouring molten metal into a holding furnace. The metal was found free from slag or any other impurities.

The methods described above are superior to the prior art methods for stirring molten metal in that they -ensure stable und uninterrupted stirring throughout the melting process, which precludes the freezing of metal in the pipes of gas pumps, as well as their failure; enhance the effectiveness of stirring and improve the quality of metal by reason of preventing its contamination with oxides and slag inclusions, which in turn, permits a gain in yield of the finished metal; - enhance the efficiency of the melting process by 20 to 25 per cent.

The illustrated apparatus for stirring molten metal 5 (Fig. 4) in a furnace melting chamber 6 comprising a refractory-lined pipe 7 fitted at one end with a detachable nozzle portion 8 which can be positioned at an angle of 0 to 900 to the pipe 7.

The nozzle portion 8 is formed of a non-wettable refractory material, the interior passage of the pipe being of any profile suitable for the purpose.

The pipe 7 is provided with a removable cover 9 which carries a nozzle 10 connected by means of a line 11 to distributor 12 for distributing pulses of compressed gas, such as nitrogen or argon, supplied from a gas vessel 1 3 of the requisite volume, which depends on the gas pressure maintained by means of a pressure regulator 14 connected to a compressed gas line. The interior passage of the pipe 7 is brought in communication with a vacuum line 15, near the end face of the nozzle 10.

To allow withdrawal of the nozzle portion 8 of the stirrer pipe 7 from the melt 5 and adjustment of its vertical position, the pipe 7 is made movable relative to the refractory-lined melting chamber 6.

The pipe 7 is received in an opening 16 of the furnace melting chamber, and, to prevent the possibility of backfire through the opening 1 6, there is provided a protective resilient ring 1 7 formed of a refractory material and secured to the wall of the melting chamber 6. The pipe 7 is connected with a shifting carrier 18 of an actuator 19 intended to enable stepwise axial movement of the pipe 7 in the body of molten metal during each operating cycle. The specific feature of the actuator 19 lies in a variable length of travel of the carrier 1 8.

In the preferred embodiment (as illustrated) the actuator 1 9 is formed as a pneumatic positioner, which makes it possible to adjust the travelling length of the pipe 7 depending upon the depth of the metal bath. It should be pointed that other suitable means, such as an adjustable stop, may be used for regulating the travelling length of the pipe 7.

There is provided an actuator 20 intended for initial positioning of the pipe 7 and connecting through a ball-and-socket joint member 21 a working member 22 with the main actuator 19, which actuator 20 makes it possible to change the pipe position in two planes.

The working chambers of the pneumatic positioner 19 are in constant communication with a compressed air line through a pressure regulator 23. Exhaust passages of the positioner are in communication with the atmosphere through regulable throttle valves 24 and distributors 25.

Such construction of the actuator 1 9 enables the pipe 7 to be moved forward and backward (upward and downward). This feature is especially important in view of the short intervals over which the pump operates.

The pump operates in the following manner.

When the bath of molten metal achieves a permissible level in the melting chamber 6, the pipe 7 is immersed into the metal bath by means of the actuator 20 so that the outlet orifice of its nozzle portion 8 is covered by the metal.

In this position, the vacuum line 1 5 is brought in communication with the interior of the pipe 7, and the gas holding vessel 13 is communicated with the compressed gas line through the distributor 12 and pressure regulator 14. The molten metal drawn in from the upper layer of the melt adjacent to the surface is raised under the action of vacuum to a desired level in the pipe 7.

Thereafter, an electromagnet of the respective distributor 25 of the actuator 19 is energized to operate the actuator 19, causing the pipe 7 to move downward to a desired depth in the melting chamber 6. On energizing the electromagnet of the distributor 12, the gas holding vessel 1 3 is brought in communication with the interior of the pipe 7 through the line 11 and nozzle 10, whereupon compressed gas is rapidly supplied to the interior of the pipe 7 from the vessel 1 3.

The pulse of gas under pressure applied to the metal in the pipe 7 causes its rapid expulsion into the bottom layer of the metal bath in the form of a high velocity jet. The discharged jet of metal is caused to move in the body of molten metal, entraining the adjoining layers thereof and thus stirring the entire volume of the metal bath. The gaseous pressure may also be applied during the downward movement of the pipe 7, i.e. during stepwise movement of the pipe 7, which at times may be very effective especially when large lumps of charge are melted.

Effective stirring of the entire body of molten metal and more rapid dissolution of alloying additions are made possible through varying both the pipe turning and sloping angles. After a pulse of compressed gas is discharged from the vessel 1 3, the electromagnet of the distributor 1 2 is deenergized., as the respective electro-magnet of the distributor 25 is concurrently (or at a certain lead) energized. Compressed gas is delivered to the vessel 13 from a gas line, while the stirrer pipe 7 is caused to travel upward for preset length. The speed of travel is adjusted by means of the respective throttle valve 24. Then molten metal is again drawn in the pipe 7, mainly from overheated upper layers of the melt.

The operating cycle of the pump is resumed in the same order.

If the depth of the melt changes, another distributor 25 of the actuator 19 is energized, the distributor being on the exhaust line corresponding to the given level of the metal bath.

Upon completion of stirring operation, the actuator 20 is operated to move the pipe 7 upwardly to its upper position, so that the pipe nozzle portion 8 is drawn out of the melt. If required, the apparatus is set in working position by lowering the pipe 7 by means of the actuator 20 to a preset level.

The apparatus described above has been found especially effective for use in melting with a variable level of the melt bath in a furnace.

Claims

1. A method of stirring a body of molten metal in a container, comprising: alternately withdrawing molten metal upwardly from the body of molten metal - the melt - in a confined space to a level above the melt and expelling the withdrawn metal into the melt under a pulse of compressed gas; the molten metal being withdrawn from an upper layer of the melt adjacent to the surface of the melt; the withdrawn molten metal being expelled into a lower layer of thge melt adjacent to the bottom of the container; the alternate metal withdrawing and metalexpelling steps being effected so that, regardless of the level of the melt, portions of molten metal are invariably withdrawn directly from beneath the surface of the melt and are discharged into the bottom region of the container.

2. A method as claimed in claim 1, wherein each portion of metal is expelled into the melt as a submerged metal jet which is projected in a fanlike fashion over an angle exceeding the angle of free flow of the metal jet and corresponding to the dimensions of the container, thereby enabling effective stirring of the melt throughout the bottom area of the container.

3. A method as claimed in claim 2, wherein, as the level of the melt rises, the jet of expelled metal is projected substantially in a fan-like fashion at a progressively increasing angle 1 to 1 2 times the angle of free flow of the metal jet.

4. A method as claimed in any of claims 1 to 3, wherein the metal-withdrawing and metalexpelling cycles are gradually decreased in frequency, in the course of stirring, from 30 to 2 cycles per minute.

5. Apparatus for stirring a body of molten metal -the melt in a container, comprising a gas pump with a stirrer pipe whose lower end is to be submerged in the melt and whose upper end is fitted with a gas nozzle, the pipe being mounted on guides for axial reciprocation therealong, an actuator for stepwise reciprocation of the pipe with a regulable speed and length of travel, and an actuator for initial positioning of the pipe within the melt and removal of the pipe from the melt.

6. Apparatus as claimed in claim 5, wherein the actuator for stepwise reciprocation comprises a pneumatic positioning device provided with discharge outlets and with regulable throttle valves adapted to communicate the discharge outlets with the atmosphere.

7. Apparatus as claimed in claim 5 or 6, including a ball-and-socket joint mounting the stirrer pipe, and an actuator for angular displacement of the pipe about the ball-andsocket joint.

8. A method of stirring molten metal, substantially as herein described with reference to the accompanying drawings.

9. Apparatus for stirring molten metal, substantially as herein described with reference to, and as shown in, Figure 4 of the accompanying drawings.