WO1996006319A1

WO1996006319A1 - Method of transferring molten metal

Info

Publication number: WO1996006319A1
Application number: PCT/AU1995/000501
Authority: WO
Inventors: Jeremy Robin Locatelli
Original assignee: Comalco Aluminium Limited
Priority date: 1994-08-18
Filing date: 1995-08-15
Publication date: 1996-02-29
Also published as: AUPM754394A0

Abstract

A method of transferring molten metal from an upper source reservoir to a lower reservoir using an inverted substantially U-shaped conduit (3) having a curved section (4) and two leg sections (7, 8). Molten metal is drawn up into the upper, generally shorter leg (7) of the conduit to partially fill the conduit. The level of vacuum in the conduit is then controlled to prevent the curved section of conduit completely filling with molten metal and provide a controlled and/or constant flow of molten metal to the lower reservoir (2).

Description

TITLE: METHOD OF TRANSFERRING MOLTEN METAL

Field of the Invention

This invention relates to the transfer of molten metal from one vessel to another in a manner by which the flow of molten metal can be precisely controlled. Background of the Invention

During the smelting and refining of aluminium and other metals, it is necessary to handle and transfer the metal in its molten, liquid state from one location to another. It is common practice to transport the molten metal in an intermediate ladle and simply pour the metal from the ladle into the receiving vessel. In the case of aluminium, this practice leads to considerable losses as the aluminium is converted to useless dross on exposure to air during the turbulent pouring operation. Another method of transferring molten metal, suggested in the prior art has been to siphon molten metal using a substantially inverted "U" shaped siphon tube. Such an apparatus and method has been proposed in International Publication No. WO86/04980 and U.S. Patent No. 4,425,932.

In such apparatus, the top of the inverted "U" of the siphon tube is connected to a vacuum source and the longer, lower leg of the "U" closed or immersed in metal contained in the receiving vessel. The shorter, upper leg of the "U" is immersed in the molten metal and the vacuum applied causing molten metal to rise up the siphon tube. The molten metal will flow from an upper source to a lower vessel if sufficient vacuum is applied to raise the metal into the inverted "U" section of the siphon tube. It is common practice to completely fill the inverted "U" section with molten metal and then maintain or isolate the vacuum source until the metal transfer operation is completed.

In such siphoning operations, the driving force effecting metal transfer is provided by the potential head of molten metal in the source reservoir above the lower receiving vessel. Thus once a siphoning action is initiated, the flow can only be controlled by restricting flow through the siphon tube (such as disclosed in WO86/04980) or by altering the head difference between the source and receiving vessels.

Furthermore, as metal is transferred it is usual for the flow rate to decrease as the relative head between the two vessels naturally reduces.

Since a particular problem with siphons used with molten aluminium is that the flow rate is heavily dependent on the diameter of the siphon tube, any blockage or restriction reduces the flow and extends the time taken to transfer metal. It has been found that over a number of operations siphon tubes gradually become blocked by dross and other impurities contained in the molten metal, requiring the tube to be removed from operation at regular intervals for cleaning.

Additionally, during a siphoning operation, unless considerable care is taken when manually controlling the vacuum, molten metal can be drawn up into the vacuum line resulting in blockage and shut-down of the system.

Therefore, whilst in theory siphoning represents an attractive means of transferring molten metal, this method has not found use in the majority of aluminium smelters, largely due to the operational difficulties relating to vacuum control and maintenance. Brief Summary of the Invention and Objects

It is an object of the present invention to provide a method of transferring molten metal which can provide a controlled flow of molten metal and a reduced frequency of cleaning.

Accordingly, the invention provides a method of transferring molten metal from an upper source reservoir to a lower reservoir including the steps of - providing an inverted substantially U-shaped conduit having a curved section and two leg sections extending therefrom, positioning said conduit such that the respective ends of said conduit are below the respective levels of molten metal in said upper and lower reservoirs and the level of the curved section of said U-shaped conduit is above the level of molten metal in the upper reservoir, increasing the vacuum within said conduit, controllmg the vacuum in said conduit to partially fill the curved section of said conduit with molten metal such that molten metal flows through said conduit from said upper reservoir to said lower reservoir and controlling the flow of molten metal through said conduit.

Preferably the legs of the conduit are of unequal lengths with the upper leg being shorter than the lower leg. Consequently, the shorter, upper leg extends below the level of molten metal in the upper reservoir and the longer, lower leg extends below the level of molten metal in the lower reservoir.

By increasing the vacuum in the conduit and in particular in the curved section of the conduit, the molten metal rises within the upper, generally shorter leg of the conduit until it begins to flow over the curved section which effectively act- as a weir. Once the molten metal has risen above this weir point it flows down the lower, generally longer leg to the conduit to the lower reservoir.

In a preferred form of the invention, the vacuum is controlled to prevent the curved section of the conduit from completely filling with molten metal, the flow of molten metal being a function of the application rate of vacuum to the conduit.

Preferably, the vacuum in the conduit is controlled by - determining a level of vacuum V-_j. at which molten metal fills the upper leg of the conduit and begins to flow over the weir point in the curved section of said conduit, increasing the vacuum in said conduit to a level less than or equal to V-,., and increasing the vacuum at an application rate R to maintain the conduit in a partially filled condition and provide a predetermined flow rate of molten metal,

The transfer of molten metal ceases when the end of the upper leg of the conduit becomes uncovered and air is drawn in, causing a sudden loss of vacuum in the conduit.

In another aspect of the invention the invention provides a method of controlling the flow of molten metal from an upper source reservoir to a lower reservoir through a conduit having a curved section and two leg sections extending therefrom, the ends of the leg sections respectively being below the molten metal levels in the upper and lower reservoirs, the method including the step of - initially increasing the vacuum within said conduit to cause molten metal from said upper reservoir to rise into the upper leg of said conduit, adjusting the rate of increase in vacuum to an application rate R which provides a controlled flow rate of molten metal into the lower reservoir without completely filling said conduit with molten metal. By maintaining the curved section of conduit only partially filled with molten metal, a controlled or constant flow of molten metal can be obtained into the lower reservoir. Brief Description of the Drawings

The features, objects and advantages of the present invention will become more apparent from the following description of the preferred embodiment and accompanying drawings in which: FIGURE 1 is a schematic diagram showing an embodiment of the apparatus for carrying out the invention, and FIGURE 2 is a graphical representation of the invention in operation.

Referring to the drawing, an upper reservoir or molten metal source 1 is shown connected to a lower reservoir 2 by conduit 3 for the passage of molten metal, between the two reservoirs. The conduit has a curved section 4 which rises above the level 5 of molten metal in the upper reservoir. The ends 7,8 of the conduit 3 are positioned below the respective levels 5,6 of molten metal in the upper and lower reservoirs 1,2.

Communicating with the interior of the conduit by a tapping 15 in the vicinity of the curved section 4 is a vacuum source including a vacuum line 9 connected to a vacuum reservoir 10. The vacuum in the vacuum reservoir 10 is maintained by a vacuum pump such as a compressed air driven vacuum pump 11. In such a vacuum pump, the vacuum is generated by the flow of compressed air through a venturi and thus may be controlled by providing a proportional control valve 12 to control the compressed air supply 14 to the vacuum pump which generates the vacuum. The proportional control valve 12 is controlled by a process controller 13 which is capable of programmable ramping and proportional control feedback from a pressure transducer 18 and pressure gauge 19 associated with the vacuum reservoir 10.

To operate the above apparatus in accordance with the invention, a vacuum is applied to the tapping 15 on the curved section of the conduit. As both ends of the conduit are submerged below the levels of molten metal in the respective reservoirs, molten metal from both reservoirs is drawn into the conduit above the level in the respective reservoirs and when the molten metal in the upper conduit leg rises above the weir point 17, the molten metal flows into the lower reservoir.

The flow of molten metal through the conduit may be precisely controlled by raising and maintaining the level 16 of molten metal in the conduit just above the weir point 17 in the curved section 4 of the conduit. A constant flow may be maintained by linearly increasing the vacuum applied to the conduit in accordance with the flow rate required, the dimensions of the reservoir and the conduit dimensions.

In the preferred embodiment the vacuum application rates and the vacuum limits are calculated in accordance with the following equations in which:

Metal Flow Required Q — kg/min Crucible Diameter d - mm Crucible Metal Depth h - mm Siphon Weir Point Height ww — mm

Density of Al @ 800°C P - 2385 kg/m³ Gravitational Constant 9.81 m/sec²

Vacuum Application Rate R = = kPafmin (1)

^■f

Start Flow Vacuum V„ = -p.* .(w-Λ)10^~* kPa (II)

Maximum Vacuum ^_MAX ⁼ ~P-g.wlO^~ ^^a G^O

By controlling the vacuum according to the above equations, the molten metal in the conduit is raised above the weir point 17 in the curved section 4 of the conduit sufficiently to maintain the required metal flow to the reservoir 2. This is accomplished without ever completely filling the conduit and thereby preventing the conduit from functioning as a conventional siphon. The applicants refer to this condition as weiring.

Under the condition where the curved section of the conduit fills completely with molten metal, weiring ceases and the system begins to operate as a conventional siphon with the flow rate governed by the head difference between the upper and lower reservoirs and the flow resistance in the pipe. A common problem associated with the siphon operation is the need for great care when manually controllmg the vacuum to avoid molten metal being drawn up into the vacuum line tapping 15 resulting in blockage of the vacuum line 9 and siphoning failure. Additionally, since the flow resistance of the siphon pipe varies in a manner which is not easily predictable due to the build-up of impurities in the pipe, the flow rate in a conventional siphon is quite difficult to control. Furthermore, the main driving force for the flow of metal through a siphon is the head difference between the upper and lower reservoir and since this difference normally decreases during the transfer cycle, so does the molten metal flow rate. Hence it is extremely difficult to achieve a controllable or constant flow during siphon operation.

The conduit will theoretically be in the weiring condition when the vacuum in the conduit is above V_CT (calculated from Equation II). The applicants have found that control of the flow rate of the molten metal is only possible whenever the conduit is weiring. Once the conduit is in the weiring condition, the molten metal flow rate can be proportionally controlled by varying the vacuum appropriately. Usually, the most desirable flow condition is a constant flow rate which can be achieved by a constantly increasing vacuum at rate R in accordance with Equation

(I)-

In accordance with the molten metal transfer method of the invention, molten metal can be transferred in an easily controlled consistent and reliable manner with the possibility of failure due to metal being drawn up into the vacuum line eliminated.

The invention also has the advantage of being able to produce constant, predictable flow during the metal transfer thereby eliminating furnace waves which disturb or upset level sensors used in furnace tilt control systems such as is required to enable charging during casting as disclosed in co-pending Australian Provisional Patent Application No. PM8300 filed 20 September 1994, the whole contents of which are herein incorporated by reference. Another advantage of the invention is that the flow through the conduit can be easily reduced or increased to suit particular conditions in the furnace, such as the need for reduced flow to minimise turbulence and dross generation when transferring molten metal into a furnace containing very little molten metal.

The applicants have found it preferable for the inside diameter of the conduit to be larger than the corresponding diameter when operating as a conventional siphon and required to transfer the same volume of metal in the same time-period. In this way, the conduit is able to operate for a longer period of time without being withdrawn from operation for cleaning. This is possible because a larger diameter conduit provides a greater margin for the build-up of impurities in the conduit bore before this build-up restricts the flow of molten metal.

In practice, the application rate of the vacuum in the curved section of the conduit is varied depending on the stage of the metal transfer. During the period when the vacuum in the conduit is between 0 and approximately 0.9 x V-_j. (as calculated in Equation 11), the application rate of the vacuum may be four times the value of R (calculated in Equation I), to save time.

The application of this high rate of increase in vacuum should preferably cease just before flow over the weir point commences to avoid the inertia effect causing a large volume of metal being drawn up into the curved section of the conduit and completely filling it. The transition to an application rate of vacuum R preferably occurs at 0.9 V-r, As the application of vacuum at rate R continues, metal flow commences at approximately V_CT (depending on the level of metal in the upper reservoir) and in normal operation will cease before the vacuum reaches V^^ (as calculated in Equation III). Metal transfer ceases when the upper reservoir empties and there is a sudden loss of vacuum as air is drawn in through the uncovered conduit inlet. If metal transfer has not ceased by the time the vacuum reaches V^^, the vacuum is maintained at the V_MAX level to avoid complete filling of the conduit. A vacuum level of V^- will only be reached if the desired flow is not achieved, due (for example) to a build up of impurities in the conduit.

A practical example of the apparatus of Figure 1 in operation is shown in Figure 2 for the transfer of molten aluminium in which :

P 2385 kg/m³ d the upper reservoir diameter 1800 π w - height of molten metal in the upper end of the conduit 1340 mm h height of molten metal in the upper reservoir relative to the end of the conduit - 530 mm

Q desired flow rate - 2000 kg min

R vacuum application rate - 7.71 kPa/min gr - vacuum at start of flow - -18.95 kPa

"MAX ~ maximum allowable vacuum - -31.35 kPa

Referring to Figure 2, the level in the upper reservoir in millimeters is represented as line 20 and commences at 530mm at time = 0 seconds. The vacuum in kPa represented by line 21 is increased at a constant high rate up to time A after which the rate of increase is lowered to an operating rate R. As the vacuum increases molten metal rises above the weir point and at time B begins to flow to the lower reservoir 2. The flow in kg/min is represented by line 22. Since the rate of increase in vacuum then remains constant, the flow of molten metal levels off and becomes constant after time C, until a sudden loss of vacuum occurs at time D, indicating that the metal transfer is compete. Line 23 represents the cumulative amount of metal transferred, in kg, as a function of time.

While the value of R, V--, and ^"V^- is determined in the above embodiment and example by the theoretical Equations I, II, III, empirical formulas may be developed and used without departing from the inventive concept defined in the claims. To develop such formulas, V-,. can be determined by detecting the commencement of flow through the conduit and the level of vacuum recorded Equations relating application rate R of vacuum after weiring commences to the flow rate Q may be developed through experimentation and trial. V^^ can also be determined through trial as the point above which the conduit begins to act as a conventional siphon.

Claims

CLΔ1MS

1. A method of transferring molten metal from an upper source reservoir to a lower reservoir including the steps of - providing an inverted substantially U-shaped conduit having a curved section and two leg sections extending therefrom, positioning said conduit such that the respective ends of said conduit are below the respective levels of molten metal in said upper and lower reservoirs and the level of the curved section of said U-shaped conduit is above the level of molten metal in the upper reservoir, increasing the vacuum within said conduit, controlling the vacuum in said conduit to partially fill the curved section of said conduit with molten metal such that molten flows through said conduit to the lower reservoir, and controlling the flow of molten metal through said conduit.

2. The method of transferring molten metal according to claim 1, wherein the vacuum in said conduit is controlled to prevent the curved section of the conduit from completely filling with molten metal, the flow of molten metal being a function of the application rate of vacuum to the conduit.

3. The method of transferring molten metal according to claim 1, wherein the vacuum in said conduit is controlled by - determining a level of vacuum V~r at which molten metal rises to the wen- point in the curved section of conduit and begins to flow through said conduit into the lower leg, increasing the vacuum in said conduit to a level less than or equal to V-,., increasing the vacuum at an application into R to maintain the conduit in a partially filled condition and provide a predetermined flow rate of molten metal, discontinuing the molten metal transfer when a large drop in vacuum is detected.

4. The method of transferring molten metal of claim 3 wherein the step of increasing the vacuum in said conduit to a level less than or equal to V-r is conducted at a rate greater than R.

5. The method of transferring molten metal of claim 4, wherein the vacuum is increased at a rate greater than R up to a level of vacuum which is about 0.9 V--,

6. The method of transferring molten metal of any one of claims 3,4 or 5 wherein the flow of molten metal is a function of R.

7. The method of transferring molten metal of claim 6, wherein the flow of molten metal is proportional to R.

8. The method of transferring molten metal of claim 7, wherein the application rate of vacuum R is constant.

9. The method of transferring molten metal of claim 6, wherein the flow of molten metal is constant.

10. A method of controlling the flow of molten metal from an upper source reservoir to a lower reservoir through a conduit having a curved section and two leg sections extending therefrom, the ends of the leg sections respectively being below the molten metal levels in the upper and lower reservoirs, the method including the step of - initially increasing the vacuum within said conduit to cause molten metal from said upper reservoir to rise into the upper leg of said conduit, adjusting the rate of increase in vacuum to an application rate R which provides a controlled flow rate of molten metal into the lower reservoir without completely filling said conduit with molten metal.

11. The method of controlling the flow of molten metal of claim 10 further including the step of - determining a level of vacuum V^ at which molten metal partially fills the curved section of conduit and begins to flow through said conduit, the vacuum in said conduit being initially increased to a level less than or equal to V--.

12. The method of controlling the flow of molten metal of claim 11 wherein the molten metal transfer is discontinued when a large drop in vacuum is detected.

13. The method of controlling the flow of molten metal of claim 11 wherein the vacuum is initially increased at a rate which is greater than the operating application rate of vacuum R.

14. The method of controlling the flow of molten metal of claim 11 wherein the flow of molten metal is a function of the application rate of vacuum R.

15. The method of controlling the flow of molten metal of claim 11 wherein the flow of molten metal is proportional to the application rate of vacuum R.

16. The method of controlling the flow of molten metal of claim 15 wherein the application rate of vacuum R is constant.

17. The method of controllmg the flow of molten metal of claim 14 wherein the application rate of vacuum R is constant.

18. The method of controllmg the flow of molten metal of claim 11 wherein the vacuum is initially increased at a rate greater than R up to a level of vacuum which is 0.9 V--.