EP0753589A1

EP0753589A1 - Melt treatment head, apparatus and method

Info

Publication number: EP0753589A1
Application number: EP96305113A
Authority: EP
Inventors: Steven Stride
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-07-12
Filing date: 1996-07-11
Publication date: 1997-01-15
Also published as: GB9514178D0

Abstract

The invention relates to a melt treatment head (29,129,229,329), apparatus (1) and method, particularly but not exclusively for the treatment of molten metals, such as aluminium.

The head is for a rotary degassing apparatus (1), and is adapted to introduce a gas to clean the melt (33), and also to carry a solid to clean and/or modify or grain refine the melt. There is also disclosed a self-contained and movable apparatus utilising the head, and a method of melt treatment utilising the apparatus.

Description

This invention relates to a melt treatment head, apparatus and method, particularly but not exclusively for the treatment of molten metals, such as aluminium.
In the description which follows, orientational terms such as raised, lowered, lower etc., refer to the apparatus in its normal condition of use, as shown in Fig.l, unless otherwise stated.
As a part of many foundry processes it is necessary to obtain optimum quality of the molten metal (hereinafter referred to as the "melt") before use. Two particular types of melt treatment are commonly necessary; cleaning, for instance, to remove oxides, inclusions and dissolved gases such as hydrogen; and modification and grain refining to improve the physical properties. The dross produced by such treatments can normally be physically removed from the melt.
Known treatments to these ends involve adding chemicals to the melt. The chemicals may be gaseous, or solid in the form of powders or tablets; if introduced as a solid, the chemicals will typically become gaseous on exposure to the melt.
Due to the noxious nature of many of these chemicals and their reaction products, as well as to obtain the best reaction, it is preferred to introduce them towards the bottom of the melt; the bubbles of gas thereafter rising through the melt.
A manually operated plunging bell which can be used to push solid chemicals towards the bottom of a melt to this end is known; however, this device involves the operator in time consuming effort and prolonged exposure in proximity to the melt.
Inert gases which may be introduced to the melt include argon and nitrogen, which act to remove hydrogen, oxides and other impurities from the melt. A widespread apparatus for introducing such gases comprises a hollow shaft connected to a rotor head; the gas is introduced along the shaft and into the melt via outlets in the head. The shaft and head are rotated at high speed (for example 650 rpm); the rotating head produces smaller bubbles than does a non-rotating head, so that the surface area for a given volume of gas is increased, and the melt treatment is thereby enhanced. The rotating head also acts to disperse the bubbles throughout the melt. Such apparatus, referred to herein as a rotary degassers, have been widely used since the early 1980's.
However, it has been found that the introduction of these inert gases does not provide optimum cleaning, so that it is desirable that a combination of inert gas and a solid flux or other treatment chemical be added to the melt. In addition, if modification and/or grain refining is required, then additional (solid) chemicals must be added.
It is known to introduce the solid chemicals in the form of a powder down a lance carrying an inert gas, such as nitrogen. Unfortunately, the high gas velocities needed to transport the powders cause turbulence and splashing in the melt which impairs dross separation and encourages air entrapment in the melt, which creates unwanted oxides. The powders also frequently block the lance.
It is also known to seek to adapt a rotary degasser to introduce solid chemicals into the melt along with the gas. However, such units face significant and similar problems to the lance in handling powders at sufficiently high gas flow rates, as well as being prone to blockage. The necessarily complex delivery means for the chemicals also renders such units expensive and physically large at a time when degassing is increasingly needed for smaller volumes of molten metal, closer to the pouring stage. Thus, whilst it has previously been known for rotary degassing to take place in large centralised units with subsequent transport and pouring into smaller units for the particular applications, this can lead to increased oxide levels due to splashing and mixing with air during pouring, and increased hydrogen levels as well as other unwanted impurities due to the delay in using the melt following its degassing.
The present invention aims to provide a melt treatment head and a method of use to introduce the desired chemicals without undue turbulence, with low pollution levels and lower dosage requirements. It is also an aim of the invention to provide a low cost, readily controllable, mobile and independent apparatus.
According to a first aspect of the invention we provide a melt treatment head for a rotary degassing apparatus, the head having a gas inlet and one or more gas outlets in communication with the inlet, characterised in that the head has a recess to accommodate a solid melt treatment product.
In this way the head can be employed in the conventional degassing mode by applying inert gas to the inlets to generate bubbles at the outlets in the melt but the head can also be used to introduce and retain solid melt treatment products, in a controlled and accurate manner.
Preferably, the recess is in the lower surface of the head, so that the melt treatment products can consequently be introduced to the desired location with the recess resisting any bouyant or other effects detrimental to accurate positioning. A single piece of equipment is consequently only needed which is not prone to blockage and significantly reduces environmental pollution.
The melt treatment head may further provide features from the following options.
Preferably the recess is concave or conical in nature. This may aid dispersion of the melt treatment products.
Channels may be provided in the head leading from the recess. Channels such as these, which may extend to the full height of the recess, aid in the sideways dispersion of the melt treatment products.
The recess may be in communication with the inlet or inlets. Again, the gas so introduced to the recess may promote dispersion. The recess to inlet connection may be relatively small, for instance less than 1/2 or even less than 1/4 of the cross-sectional area of the outlets, so as to control the proportion of gas leaving via the outlets as against via the recess.
The outlets may be radially aligned or angled downwards to give the necessary bubble conditions and aid drainage of the passages on removal from the molten metal. The head may be configured to provide recessed and/or extended portions. These may serve to provide a chopping action encouraging dispersion of the bubbles and/or melt treatment products.
The recess may be provided with melt treatment product restraining means. Complementary elements in the recess and on the melt treatment product form may be provided to ensure accurate alignment and retention.
The head may be mounted on a shaft component down which the gas is passed.
According to a second aspect of the invention we provide a rotary degassing apparatus incorporating a head as herein defined, the head being connected to the apparatus by a hollow shaft, the apparatus including (i) means to raise and lower the shaft and head, (ii) means to rotate the shaft and head, and (iii) means to introduce gas through the shaft and into the head.
The apparatus can also have one or more of the following features:-

a) a self-contained power supply;
b) a self-contained gas supply;
c) the apparatus is movable;
d) the degassing head is mounted on a movable and counterbalanced portion of a support frame;

Each of these features alone and in combination bring advantageous properties to the device in terms of safety, convenience and control.
Preferably the degassing element is manually movable.
The gas supply may comprise a compressed gas cylinder.
The power supply may preferably comprise a battery or mains supply, as required.
The counterbalanced mounting may comprise a depending arm extending from the support frame. Preferably the arm is controlled by a releasable latch. The control is preferably effected by a manually operated handle.
Preferably the apparatus is rendered movable by wheels, most preferably provided on a base member.
Most preferably the controls for the apparatus are mounted to the side, that is relative to the direction the depending arm extends in, so as to provide the operator with better access and visibility of the melt during operation, rather than obscuring the melt by the support frame etc.
According to a third aspect of the invention we provide a method of treating molten metal comprising the steps of:

1) introducing solid melt treatment product between the surface of the melt and an undersurface of the head
2) lowering the head into the melt to convey the melt treatment product into the melt
3) introducing gas through the head into the melt whilst the melt treatment products are being dispersed from the head into the melt.

In this way a more controlled and effective introduction of melt treatment products is provided.
Preferably the element is rotated. Preferably the speed of rotation is varied. Thus various stages of the method can employ differing speeds. In a particularly preferred method the speed of rotation is lower, or even zero, during an initial stage in which the solid melt treatment product is being dispersed in the melt, and is increased during the subsequent degassing stage.
Gas may be introduced through the head during the whole process or only during degassing. The flow rate may be varied as required. For instance the gas may be used to disperse the melt treatment product prior to its use for degassing.
According to a fourth aspect of the invention we provide a melt processed by the apparatus and/or method and/or head of the previous aspects or an article produced from such a melt.
Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:-

Fig.1: is a perspective view of an embodiment of the apparatus in use;
Fig.2a: is a sectional view of a first rotary head embodiment;
Fig.2b: is a side view of the head of Fig.2a;
Fig.2c: is a view from above of the head of Fig.2a
Fig.3a,b,c: are views corresponding to those of Figs. 2a,b,c of a second rotary head embodiment;
Fig.4a,b,c: are views corresponding to those of Figs. 2a,b,c of a third rotary head embodiment; and
Fig.5a,b,c: are views corresponding to those of Figs. 2a,b,c of a fourth rotary head embodiment.

As shown in Fig.1 the apparatus 1 comprises a support frame 3 mounted on a platform 5 which can readily be moved around by means of wheels 7a,7b of which the rear pair 7b at least are capable of changing direction. The upright support 9 carries a cross member 11 upon which the shaft 13 is rotatably mounted. A control unit 15 is provided to provide "on" and "off" and variable speed control for the rotation of the shaft 13 and also to control the rate of gas introduction into the shaft 13. The inert gas is supplied by a self-contained source, in the form of a conventional compressed gas cylinder 17, mounted on a recess 19 accommodated within the base of the apparatus.
A substantially horizontal frame 21 provides handles 23 at one end to control the movement of the apparatus 1 and a bar 25 to retain the cylinder 17 in the upright position. The opposing end of this frame 21 carries a protection shield 27 which substantially encloses the shaft 13 and head 29 during movement between crucibles or furnaces 31 to be treated, i.e. the head 29 is substantially surrounded by the shield 27 when in its raised position as shown in Fig.1. It will be understood, however, that the shield 27 will also provide some protection for the head 29 and part of the shaft 13 when the head is lowered into the melt 33. In alternative embodiments, the frame 21 can also carry a cowl depending downwardly from the shield, which cowl can serve to control the emission of noxious gases which may be given off from the surface of the melt during treatment.
A handle 35 provided on the upright frame 9 is used to lower the cross member 11 carrying the shaft 13 into, and raise it out of, crucible 31 contained within standard support and heating apparatus 36. The cross member 11 is connected to a counterbalance within the upright support 9 such that an operator can easily and readily raise and lower the cross member 11, and thus control the positioning of the head 29 and shaft 13 relative to the crucible 33.
The handle 35 carries a spring-biassed lever 37, which lever is connected (suitably by Bowden^TM cable) to a peg (not shown) which is selectively locatable in one of the spaced apertures 39 in the upright support 9. Thus the peg and apertures together comprise a latch mechanism, and enable the operator to position the cross member 11 relative to the upright support 9. When it is subsequently desired to raise or lower the cross member 11, the lever 37 is pulled towards the handle 35, which releases the peg from its aperture.
Power for the shaft 13 is provided by means of an electric motor carried within cross member 11. The motor can be coaxial with the shaft 13, and be in direct driving engagement therewith. However, in the preferred embodiment shown the motor is located adjacent the upright support 9 and drives the shaft by means of a toothed belt engaging a spindle on the shaft 13.
A 24 volt DC battery supply 41 accommodated on the base 5 of the apparatus 1 can provide an electrically safe power source without any need for trailing cable.
The control unit 15 can be used to determine the rotational speed of the shaft 13. In the preferred embodiment shown, the rotational speed is variable between a number of predetermined rotational speeds conforming to the desired rotational speeds for the various stages of operation of the apparatus. The predetermined rotational speeds may differ for different crucible sizes and for different melt materials. In an alternative embodiment the rotational speed is infinitely variable between 0 and 850 rpm.
As with conventional systems the shaft 13 and head 29 are typically made from graphite, fused silica or other refractory materials.
As illustrated in figs.2a,b and c, the shaft 13 is hollow, and at least the lower part of the shaft as shown is formed integrally with head 29. The inert gas can be introduced from cylinder 17 along central inlet 45 in the direction of arrow A, in known fashion. The inlet 45 communicates with diverging outlets 47 which serve to introduce the gas to the melt 33.
The undersurface 49 of the head 29 is provided with a concave recess 51. This recess 51 is designed to restrain a sachet, tablet or other form of solid melt treatment product (not shown) during introduction to the melt 33 and to retain it in the desired position within the melt during its dispersal. For example, if the melt is aluminium, the solid treatment product may be in the form of a powder wrapped in aluminium foil, the resulting sachet being bouyant relative to the melt; the aluminium foil is thick enough so that it does not itself melt and release the powder before the operator has had time to move the head to push the sachet to its desired location within the melt.
The melt treatment product may be a flux or other chemical adapted to clean the melt, or it may be a modifying or refining chemical, or it may be a combination of both of these. Also, the melt treatment product may include metallic sodium and/or alloy additions such as magnesium.
Thus, in use the apparatus with its integral gas and power supply can be brought to the desired location with the head 29 and shaft 13 in the raised position as shown in Fig.1. Once correctly positioned over the melt 33 the sachet, tablet or other form of melt treatment product can be floated on the melt surface by the operator directly under the head 29. The operator can then, using the counterbalance cross member 11, lower the head 29 onto the tablet etc., checking all the time for correct alignment. Given correct alignment the recess 51 under the head 29 retains the treatment product and pushes it under the surface of the melt and to a position controlled by the extent to which the head 29 is lowered. The cross member 11 can then be latched into position by the operator.
After a desired period of time, for instance 15 seconds, the operator can use control unit 15 to start slow rotation of the head 29 so as to to encourage dispersal of the treatment product within the melt. Over time, as the melt treatment product is dispersed, the rotational speed can be increased to full degassing speeds, for example 650 rpm.
Gas will typically be introduced along shaft 13 during the initial stage of the process during which the melt treatment product is being dispersed though the melt, though this may be at a slower rate than during the later stages of the process, as desired for the particular application.
Figs.3a,b and c illustrate a second embodiment of head 129 whereby the head is provided with four outlets 147 readily connected to the gas source via inlet 145 and in which the recess 151 in the underside of the head is also connected to the gas supply by section 153. In this example the recess has a conical form.
Figs.4a,b and c illustrate a third embodiment of head 229 in which the outlets 247 have their outer limit in an annular groove 255 passing around the head. Portions of the head 229 above this groove have been removed to create projections 257 which develop a chopping action on the gas stream promoting smaller bubbles, and so increasing the surface area for a given volume of gas. The recess 251 in the head is again concave in this embodiment.
Figs.5a,b and c illustrate a fourth embodiment of head 329 in which the inlet 345 is in communication with three outlets 347, the outlets being arranged at 120° spacings. The recess 251 under the head is again concave in this embodiment. The recess 351 is in communication with three channels 359, the channels 359 being arranged at 120° spacings. The outlets 347 and the channels 359 do not intersect in this embodiment.
In the embodiment of Fig.5, the recess 351 is approximately 45mm deep at its centre and has a maximum diameter of approximately 110mm. Such a recess has been found to be suitable to retain 300 grams of powdered melt treatment product which has been wrapped in aluminium foil into a sachet approximately 100mm in diameter and approximately 50mm deep. The same quantity of melt treatment product which is in tablet form will typically occupy a smaller volume and so can also be retained by the recess of Fig.5, as will tablets or sachets of smaller quantities of melt treatment product. Should larger quantities of melt treatment product be desired, then a larger recess can be provided, though at the addition rates possible with this invention 300 grams of melt treatment product is believed to be suitable for treating the melt which can be accommodated in most of the crucibles of which we are presently aware.
In addition to the embodiments shown, further alternative embodiments could be provided. One such alternative is similar to the embodiment of Figs. 2, but includes radial channels leading from the recess to the cylindrical surface of the head. The channels can be open to the undersurface of the head.
In another alternative embodiment, the gas inlet is connected to an outlet in the recess, so that all of the inflowing gas enters the recess. The recess has upwardly directed channels (similar to the channels 359 of Fig.5) through which the gas subsequently flows, carrying melt treatment product from the recess and into the melt.
In addition, the preferred embodiments shown assume that the melt treatment product is bouyant, so that the recess is formed in the lower surface of the head to retain the product under the surface of the melt. It will be understood, however, that in other but less preferred embodiments the recess could be in another surface of the head, or the head could carry temporary or substantially permanent restraining means for the products. In one other embodiment, the melt treatment product is restrained in the recess by aluminium wires or tapes passing around the sachet or tablet and through one or more of the outlet channels. In such an embodiment, the melt treatment product can be affixed to the head before the head is moved over the melt, so that the correct positioning is assured without the operator perhaps having to move the apparatus, or else move the melt treatment product on the surface of the melt. It is arranged that the melt treatment product is dispersed into the melt, and the restraining means melts, before the head is rotated at full degassing speed, so that the balance of the head is not adversely affected.
The carefully controlled introduction of the treatment product in accurately known quantities and the gradual introduction of degassing ensure optimum properties for the melt not only in terms of chemical modification but also in terms of degassing. This is achieved without introducing unnecessarily high levels of noxious substances into the environment around the apparatus and without causing undue turbulence within the melt which can produce inclusions and oxides, as well as extending the processing time.
Once degassing has been completed, rotation of the head 29,129,229,329 is stopped and it can then be raised out of the melt ready for use with another batch, or it can be moved to another location for use with another crucible. The molten metal can be transferred to the pouring station or other unit for use in its optimum condition, i.e. with minimum delay.
The self-contained and highly portable nature of the apparatus 1 make it particularly suitable for use on small volumes of molten metal close to the end-use location. As above described, use on small volumes close to the end-use location assist in ensuring that the melt is in its optimum condition at the point of use.
In addition, it has been found that the procedure gives particularly excellent mixing and distribution of the treatment product within the melt. Secret trials have shown that less treatment flux is needed to achieve the desired gas levels in the melt using the present technique than with conventional techniques. For instance, in comparative tests, 50 grams of flux introduced by the technique of the present invention were sufficient to reduce gas levels to the specification for which 750 grams of flux were previously introduced through a flux injector. A gas level of 0.04 cm³/100g aluminium was achieved in the test.
Flux addition rates in general down to 0.05% of melt weight have been shown to produce dry easily removable dross. The use of smaller quantities of flux is particularly advantageous as not only does this represent a cost-saving in terms of flux consumption but also significantly reduces fume emissions to negligible levels.
The technique of the present invention has also been demonstrably advantageous in reducing hydrogen gas levels below 0.1 cm³/100g aluminium compared against 0.4 in the prior art techniques, and also to reduce oxide levels down from one turn to two and a half turns of the fluidity spiral test apparatus commonly used. Fluidity is reduced significantly by the presence of oxides. The reduction in oxide content in a casting or moulding is particularly desirable as oxides cause crack generation particularly in highly stressed parts such as internal combustion engine pistons.
In further tests, necessary addition rates of tablets to achieve the desired conditions of 200 grams were found to be sufficient using the present invention as against manufacturers recommendations of 300 to 600 gram tablets for the 300 kg melt concerned using prior art techniques.
The present invention thus not only represents a significant improvement in terms of capital costs and user-friendliness but also provides for advantageous advances in treatment times and in achieving removal of dissolved hydrogen oxides and inclusions within the melt.

Claims

A melt treatment head (29,129,229,329) for a rotary degassing apparatus (1), the head having a gas inlet (45,145,245,345) and one or more gas outlets (47,147,247,347) in communication with the inlet, characterised in that the head has a recess (51,151,251,351) to accommodate a solid melt treatment product.
A melt treatment head according to claim 1 characterised in that the recess is in the lower surface (49) of the head.
A melt treatment head according to claim 1 or claim 2 characterised in that the recess (151) of the body is in communication with the gas inlet (145) whereby gas can be introduced into the recess to promote dispersion of the melt treatment product.
A melt treatment head according to any of claims 1-3 characterised in that at least one channel (359) extends from the recess (351).
A rotary degassing apparatus (1) incorporating a head (29,129,229,329) according to any of claims 1-4, the head being connected to the apparatus by a hollow shaft (13), the apparatus including (i) means (35,37) to raise and lower the shaft and head, (ii) means (15) to rotate the shaft and head, and (iii) means (17;45,145,245,235) to introduce gas through the shaft and into the head.
An apparatus according to claim 5 characterised in that the means to rotate the head (15) can be operated at a number of differing rotational speeds.
An apparatus according to claim 5 or claim 6 characaterised in that the shaft (13) is connected to a counterbalanced cross member (11), and in that handle means (35,37) are provided to raise and lower the cross member.
An apparatus according to any of claims 5-7 characterised in that the apparatus is mounted upon wheels (7) so as to be movable, in that the apparatus carries its own power source (37), and in that the apparatus carries its own gas supply (17).
A method of treating molten metal with an apparatus (1) according to any of claims 5-7 characterised by the steps of positioning solid melt treatment product between the surface of the melt (33) and an undersurface (49) of the head (29,129,229,329), lowering the head into the melt to convey the melt treatment product into the melt, and introducing gas through the head into the melt whilst the melt treatment products are being dispersed from the head into the melt.
A method according to claim 9 in which the head (29,129,229,329) is either stationary or is rotated at a low speed during an initial stage of treatment in which the melt treatment products are being dispersed into the melt, and is rotated, or is rotated at a higher speed, during a subsequent stage of treatment.