ROTARY DEVICE FOR TREATING MOLTEN METAL

Abstract

A rotor for use in the treatment of molten metal. The rotor comprises a roof having a central axis and comprising a plurality of peripheral cut-outs. An intermediate plate extends axially from the roof and comprises a plurality of sides having arcuate portions. A plurality of blades extend axially from the intermediate plate. The rotor is intended for use in casting operations, particularly in the treatment of non-ferrous metals.

Claims

1. A rotor for use in the treatment of molten metal, the rotor comprising: a roof having a central axis and comprising a plurality of peripheral cut-outs; an intermediate plate extending axially from the roof, comprising a plurality of sides having arcuate portions; and a plurality of blades extending axially from the intermediate plate, characterised in that the intermediate plate is directly adjacent to, and contiguous with, the roof, wherein the arcuate portions of the plurality of sides are concave such that the centre of each arcuate portion is closer to a central axis of the rotor than the ends of the arcuate portions.

2. The rotor according to claim 1, wherein the plurality of peripheral cut-outs comprises at least six cut-outs.

3. The rotor of claim 1, wherein the intermediate plate is located between the roof and the blades.

4. The rotor of claim 1, wherein the intermediate plate and/or the blades extend up to the edge of the roof.

5. The rotor of claim 1, wherein the roof and intermediate plate comprise a central aperture therethrough for fluid communication with a fluid supply.

6. The rotor according to claim 5, wherein the rotor comprises a chamber defined axially by the intermediate plate and radially by an internal surface of the blades, and wherein central aperture opens into the chamber.

7. The rotor according to claim 6, wherein the chamber has a width or nominal radius greater than the width or radius of the central aperture.

8. The rotor of claim 1, wherein the intermediate plate comprises three sides connected by three ends or corners, and wherein each end or corner comprises at least one of said plurality of blades.

9. The rotor of claim 1, wherein the plurality of sides of the intermediate plate each comprise a pair of straight portions separated by one of said arcuate portions.

10. The rotor of claim 1, wherein the blades have a cross sectional shape the same as the adjacent portions of the intermediate plate.

11. The rotor according to claim 1, wherein the peripheral ends of the blades are tapered and form a pointed, flat or rounded edge.

12. The rotor of claim 1, wherein the rotor has C3 rotational symmetry.

13. The rotor of claim 1, wherein the roof, intermediate plate and plurality of blades are integrally formed such that the rotor is contiguous.

14. A rotary device comprising the rotor according to claim 1, and a shaft, and wherein the rotor is provided at one end of the shaft.

15. The rotor according to claim 1, wherein rotor and/or the rotary device is formed from an isostatic pressed refractory material.

16. A method of treating molten metal comprising the steps of: immersing the rotor, and optionally part of the shaft, of the rotary device according to claim 14 into the molten metal, rotating the rotor, and passing one or more molten metal treatments through the rotary device.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0039] Embodiments of the invention will now be described with reference to the following drawings, in which:

[0040] FIG. 1 is a perspective view of a rotary device;

[0041] FIG. 2 is a perspective view of the rotary device of FIG. 1;

[0042] FIG. 3 is a side view of the rotary device of FIGS. 1 & 2;

[0043] FIG. 4 is a graph comparing the torque of two rotor designs; and

[0044] FIG. 5 is a graph comparing the degassing efficiency of two rotor designs.

DETAILED DESCRIPTION OF THE INVENTION

[0045] FIGS. 1 to 3 show a first embodiment of a rotary device 10. The rotary device 10 comprises a rotor 11 connected to a shaft 20. The shaft 20 is tubular and has a central passageway 16A extending the length thereof. The rotary device 10 defines an axis A which extends centrally through the length of the shaft 20, central passageway 16A and the rotor 11. The shaft 20 has a flared end where it connects to the rotor 11 with a curved surface to ensure a smooth transition between outer surface of the shaft 20 and the rotor 11.

[0046] The rotor 11 is formed in approximately three layers, with a roof 12 adjacent to the shaft 20, an intermediate plate 13 extending axially from the roof 12, and three blades 15 extending axially from the intermediate plate 13. The roof 12 is approximately disc shaped (i.e. it has an approximately circular shape in the horizontal cross-section) and has a thickness in the axial direction A. The roof 12 is provided with a series of cut-outs 18 in its outer (i.e. peripheral) surface which extend axially through the thickness of the roof 12. The cut-outs 18 are arcuate and have a curved cross-section when viewed axially and a depth in the radial direction relative to axis A. In other words, the cut-outs 18 are arcuate and have a curved shape in the horizontal cross-section and a depth in the radial direction relative to axis A.

[0047] As best shown in FIG. 2, the intermediate plate 13 has an approximately triangularly shaped cross-section formed from a series of sides 13a which join at corners 13b. Each side 13a has a pair of straight portions 13c separated by arcuate portions 14 extending radially into the sides 13a thereof. The peripheral ends (i.e. the radially outermost ends) of the blades 15 and the intermediate plate 13 taper to a sharply pointed edge or corner 13b. In the depicted embodiment, the intermediate plate 13 is sized such that the corners of the plate 13b are positioned at the peripheral edge of the roof 12, although in other embodiments, the corners may be spaced away from the peripheral edge of the roof 12. The central aperture 16 opens to the centre of the intermediate plate 13, thus providing a continuous passage through the rotor 11 from the shaft 20.

[0048] From each corner 13b of the intermediate plate 13 is a blade 15, which extends axially away from the intermediate plate 13 and roof 12. The blades 15 have a cross-section area (as seen in the horizontal cross section) equal to the immediately adjacent region of the intermediate plate 13, such that the blades 15 extend continuously from the intermediate plate 13. The shape of the blade 15 is thus defined by the corners 13b, straight portions 13c and part of the arcuate portions 14 of the intermediate plate 13. The surface of the blades 15 closest to the central axis A is curved and partly defines a round chamber 17 beneath the intermediate plate 13 and between the three blades 15. The chamber 17 is thus open through spaces between the respective blades 15.

[0049] The corners 13b and leading edge of the blades 15 have an angle of 60 such that the straight portions 13c are coplanar. In alternative embodiments (not shown), the corner angles may be different, such that the straight portions 13c of each pair are angled relative to each other. Similarly, the size of the arcuate portions 14 can be provided in a range of lengths and depths (i.e. radially).

[0050] The rotor 11 and shaft 20 are made from refractory materials and can be formed by isostatic pressing or by moulding (including casting). The rotary device 10 shown in FIGS. 1 to 3 is formed as a single integrally formed component. In alternative embodiments, the rotor 11 and shaft 20 are formed as separate pieces and connected together. The shaft 20 can be provided with a connecting portion, such as a screw-threaded portion or push-fit arrangement. In such embodiments, the rotor 11 may be provided with a socket with a corresponding connecting portion, such as a corresponding screw threaded portion or push-fit socket.

[0051] In use, the rotary device 10 can be secured in a Rotary Degassing Unit having a motor and a gas supply, and inserted into a container of molten metal. The gas can be passed down the hollow shaft, through the rotor 11 and the central aperture 16 and into the chamber 17, while the rotary device 10 is driveable by the motor to rotate about the axis A. The rotation of the rotor 11 disperses the gas blown into the chamber 17 as the gas rises through the molten metal. Without wishing to be bound by theory, it is believed that the peripheral cut-outs 18 in the roof 12 act upon the gas bubbles which rise through the molten metal. In particular, peripheral cut-outs 18 collect the bubbles as they rise from the chamber 17 and, due to the rotation of the rotor 11, throws the bubbles radially from the rotor 11. This leads to an especially effective distribution of the bubbles throughout the molten metal. Furthermore, the chamber has been found to increase the retention time of gas bubbles below the rotor e.g. the internal faces of the blades slow and prevent some of the bubbles from being thrown radially b the rotation of the rotor. This is believed to increase the mixing between the gas and the molten metal, and thus improves the purging effect of the gas upon the molten metal.

Water Modelling Results

[0052] The performance of various rotor designs was tested by water modelling, in a full size crucible fitted with a baffle plate. The crucible was filled with 220 litres of water at 16 C. and the rotor immersed until 190 mm from the bottom of the vessel. Water has similar viscosity characteristics to molten aluminium, and is therefore a useful proxy to indicate the performance of a rotor in molten metal.

[0053] Two rotor designs were compared: (A) a design according to the invention as shown in FIGS. 1 to 3 and (B) a commercially available rotor by the present applicant (the XSR rotor) as described in WO2004/057045.

Stirring Power

[0054] Torque measurements were carried out at different rotation speeds, to compare the relative stirring power of each rotor design. The experiments were repeated at least three times in total and a mean average value calculated.

[0055] The torque measurement results are shown in FIG. 4, which is a graph of torque (N.Math.m) vs rotation speed (rpm). At all rotation speeds, rotor designs A exhibited higher torque than comparative design B. Greater torque has been found to lead to smaller bubbles of gas being introduced through the rotary device. The smaller bubble sizes lead to an increase in surface area and lower rising speed of the gas bubbles which increases the gas residence time in the melt and thus improved degassing of the liquid metal in use.

Degassing Efficiency

[0056] An oxygen meter was immersed in the water, towards the top of the crucible. Rotor designs A and B were each rotated at 300 rpm and 400 rpm and the time taken for the oxygen level to reach a minimum plateau was measured. Oxygen dissolved in water exhibits similar behaviour to hydrogen dissolved in molten aluminium, so this test gives a useful measure of degassing efficiency in molten metal.

[0057] The degassing results are shown in FIG. 5, which is a graph of oxygen level (mg/L) vs time(s). The rotor design according to the present invention (Ex. A) exhibited significantly faster oxygen removal from the water than comparative Ex. B at both 300 rpm and at 400 rpm. An improved degassing rate is desirable for reducing treatment times and increasing throughput at the foundry. Furthermore, the improved rate at lower rotational speeds is particularly desirable for minimising the wear on both the rotary devices but also the rest of the rotary degassing units, thus minimising maintenance costs and downtime.

Aluminium Melt Testing Results

Visual Observations

[0058] The rotary device was immersed in 400 kg liquid aluminium at 720 C. to a depth of 200 mm from the bottom of the vessel. A baffle was fitted to the degassing unit adjacent to the shaft of the rotary device, gas was supplied at varying flow rates through the rotary device, and the rotor was rotated at varying speeds. Visual observations of the melt surface were recorded in Table 1 to determine acceptable working conditions for the rotary device.

TABLE-US-00001 TABLE 1 Rotation Gas flow rate speed 10 l/min 15 l/min 20 l/min 100 rpm Large bubbles Large bubbles 150 rpm Large bubbles Large bubbles 200 rpm Good melt surface Large bubbles Large bubbles 250 rpm Good melt surface Acceptable Acceptable surface surface 300 rpm Very good melt Acceptable Good melt surface surface surface 350 rpm Very good melt Good melt Good melt surface surface, small surface vortex around shaft 400 rpm Very good melt Good melt Acceptable, large surface, small surface purge gas amount is vortex around shaft well distributed 450 rpm Very good melt Small Acceptable, large surface, vortex turbulences, purge gas volume is around shaft but acceptable well distributed 500 rpm Turbulent, vortex, Small Acceptable, small but acceptable turbulences, vortex but acceptable

[0059] The visual observations determined that the working window for the rotor is between 250 and 450 rpm with a gas flow rate through the rotor of 10-20 l/min. Typically, a relatively calm melt surface is desirable to avoid negative effects. Large bubbles are indicative of poor mixing and low bubble surface area and thus gas efficiency. Turbulent surfaces are also more likely to re-dissolve impurities which have floated out of the melt. A small vortex can lead to faster degassing rates and better mixing efficiency, but larger vortexes lead to greater air and oxide entrainment, and thus a balance must be struck for greatest efficiency.

[0060] The working window for the rotary device utilises lower rotation speeds and lower gas consumption than existing commercial rotary devices. The lower rotation speeds are desirable due to reducing the wear on the RDU and on the rotors themselves, thereby increasing the working life of the rotary device.

Degassing Efficiency

[0061] Reduced pressure testing (RPT) can be used to identify the density index (DI) of a metal sample. RPT is an inexpensive and effective way to determine hydrogen levels in aluminium and thus control the gas porosity. A sample of the aluminium is taken from the melt and immediately placed under a vacuum dome of a Reduced Pressure Tester. The sample is allowed to solidify under vacuum for approximately 4 minutes (i.e. at 8 kPa of pressure). Solidifying under vacuum expands the volume of hydrogen gas approximately ten times greater than solidification at normal atmosphere allowing measurement and evaluation of gas levels in the melt.

[0062] To test the hydrogen degassing efficiency of the rotor in liquid aluminium, a series of tests were carried out. The rotary device was immersed in 400 kg liquid aluminium at 720 C. to a depth of 200 mm from the bottom of the vessel. To act as a baseline, the liquid aluminium was first upgassed for 4 minutes with a combination of 15 l/min of a mixed gas comprising 30% hydrogen and 5 l/min inert gas and with a rotor speed of 400 rpm. The aluminium was then degassed using the rotary device according to the rotation speeds and gas flow rates in Table 2. A sample was withdrawn, the density index (DI) was calculated, and the process repeated under new degassing parameters. The testing was repeated using a new aluminium sample and the average recorded in Table 2. The DI of the upgassed aluminium was measured periodically and the average calculated to be 12.6%.

TABLE-US-00002 TABLE 2 Gas flow rate Rotation speed 10 l/min 15 l/min 20 l/min 200 rpm 4.6% 250 rpm 1.7% 0.2% 0.4% 300 rpm 0.5% 0.1% 1.3% 350 rpm 1.1% 0.2% 0.2% 400 rpm 0.8% 0.0% 0.2% 450 rpm 0.0% 0.2% 0.2% 500 rpm 0.3% 0.2% 0.2%

[0063] The density index (%) is calculated using the formula:

[00001]$\mathrm{DI}=\frac{{}_{\mathrm{atm}}-{}_{8\mathrm{kPa}}}{{}_{\mathrm{atm}}}$

where .sub.atm and .sub.8 kPa are the densities of the samples measured in g.Math.cm.sup.3 solidified at atmospheric and 8 kPa pressure respectively.

[0064] The data shows that the rotor was found to be highly effective at degassing the aluminium melt, even at low rotation speeds and low gas flow rates.

[0065] The inventors have found that the rotors of the invention are as effective or more so compared to commercially available rotors at lower rotation speeds and/or lower gas consumption. Without wishing to be bound by theory, it is believed the arcuate portions of the intermediate plates are effective at increasing the area of the plate and of better accelerating the melt and better distributing the gas-melt mixture. The blades and the cut-outs in the roof are also believed to increase the torque imparted by the rotor. As noted previously, greater torque is believed to produce smaller bubbles of gas with a higher surface area and thus improving degassing efficiency. The combination thus provides a rotor with high torque, which produces bubbles with a longer residence time, and which is also very effective at mixing the melt.

[0066] Furthermore, the rotors are easier to manufacture than commercially available rotors, which require 3-axis machining and are thus limited to materials such as synthetic graphite. The simple design can be produced, for example, by isostatic pressing or moulding/casting without requiring complex machining processes, and in some embodiments, using 2-axis machining. This permits the use of alternative materials such as clay graphite or castable refractory materials which are far more durable than synthetic graphite. The result is an increase in rotor lifetimes and a decrease in maintenance leading to productivity improvements.