Patent Application
20110139267
This application claims priority to and the benefit of Canada Patent Application No. 2,688,730, filed Dec. 16, 2009, which application is incorporated herein by reference in its entirety and made part hereof.
This application relates to the siphoning of liquids, in particular to any liquid having a supernatant liquid. In the context of aluminium production, a particular application relates to the siphoning of molten aluminium from electrolysis cells, in which there is a supernatant liquid electrolytic bath.
A problem which occurs during siphoning of aluminium is that the metal layer has a limited depth and the volumetric flow during siphoning is difficult to control. While it is desirable on the one hand to increase production flow rates and to minimize blockages, if the flow is not optimized, a poorly managed flow rate can result in bath being entrained with the metal. Bath entrainment has many negative effects on electrolytic cell processing and should ideally be minimized.
Siphoning of aluminium metal from an operating electrolytic cell is usually done with a crucible which is positioned with a crane or a suitable vehicle. The crucible has an integral siphon which is inserted into an electrolytic cell at the depth of the metal. Once positioned, a vacuum is induced into the crucible, usually using an air ejector whereby the metal is aspired through the siphon. The air flow through the air ejector is controlled manually using a valve on a compressed air supply.
In practice, a stable metal flow is not obtained and very large fluctuations can be observed during siphoning of a single cell and of adjoining cells.
Some of the factors which can explain some of the variations in flow rate are, for example: the position of the crucible relative to the metal/bath interface; any obstructions limiting free flow of metal into the siphon inlet such as surface variations on the floor of the electrolytic cell or lumps of solidified bath; variations in air temperature during siphoning; variations during siphoning in how well the crucible is sealed; variations in air pressure supply; crusting of siphons from bath entrainment, to name a few.
Accordingly, there is a need to provide means for siphoning a liquid, aluminium in particular, with a pre-defined metal flow rate while minimizing fluctuations to the flow rate.
In accordance with one aspect, there is provided a system for controlled siphoning of a liquid to be transferred at a pre-determined target flow rate Q into a container having a siphon communicating with a headspace for the container, the system having: an air ejector coupled to a source of compressed air and in fluid communication with the headspace; weighing means for measuring the weight of the container during siphoning; a control unit operatively connected to said weighing means for receiving weight measurements and calculating instantaneous liquid flow rates q of liquid being siphoned into the container, the control unit having an output providing target vacuum set points V for continuously adjusting a flow rate of the compressed air flowing through the air ejector such that the actual flow rate q of the liquid being drawn into the container generally corresponds to said target flow rate Q; and a vacuum regulator provided between the source of compressed air and the air ejector, and operatively connected to the control unit and a vacuum feedback pressure line from the headspace for regulating the flow of compressed air delivered to the air ejector in response to vacuum pressure changes in the headspace and to the target vacuum set-points V fed to the vacuum regulator by the control unit.
In accordance with a further aspect, there is provided a system for controlled siphoning of a liquid to be transferred at a pre-determined target flow rate Q into a container having a siphon communicating with a headspace for the container, the system having: an ejector operatively connected to a source of motive fluid, the ejector being in fluid communication with the headspace to cause liquid to be drawn into the container through the siphon as the motive fluid flows through the ejector, a flow regulator provided between the source of motive fluid and the ejector for regulating the flow of motive fluid fed to said ejector, thereby providing for the adjustment of a vacuum pressure at said headspace; a sensor for sensing an operating parameter indicative of an actual flow rate (q) of the liquid drawn into the container; and a control unit adapted to: use the operating parameter from the sensor to determine an adjustment value of a flow rate of the motive fluid for the actual flow rate (q) to generally correspond to the predetermined target flow rate (Q), and controlling the flow regulator to regulate the flow rate of the motive fluid in accordance with the adjustment value.
In accordance with a still further aspect, there is provided a valve actuator adapted to be coupled to a valve assembly having a housing defining a primary inlet and an outlet, a valve member received in said housing for regulating a fluid flow between the primary inlet and the outlet, and a valve stem extending from the valve member through the housing; the valve actuator having a vacuum chamber sealed from a pressure chamber, the vacuum chamber having a vacuum chamber inlet and a first diaphragm responsive to fluid pressure in said vacuum chamber, the first diaphragm being coupled to the valve stem, the pressure chamber having a secondary inlet and a second diaphragm responsive to fluid pressure in said pressure chamber and coupled to said valve stem, both said first and second diaphragms being adapted to move the valve stem to control the position of the valve member and determine the fluid flow through the outlet of the valve assembly.
A system for controlled siphoning will now be described with reference to the schematic drawing of
Siphoning takes place when compressed air is allowed to flow through the air ejector 22 to atmosphere. The air ejector 22 has a suction port coupled to a headspace 30 in the crucible 20 and thereby induces a vacuum which changes according to variable conditions, as previously described. A siphoning tube 32 operatively connected to the headspace 30 draws liquid metal 34 from an electrolytic cell (not shown) and fills the crucible 20 with a predefined mass of metal. The mass of metal to be siphoned is predefined in accordance with standard operating procedures and will depend on the production levels of the electrolytic cell and minimum metal levels required to maintain a cell in operation. The incremental mass of the crucible 20 may be determined by a scale 36 coupled to a crane 38 from which the crucible 20 is suspended, in use. Other types of weighing devices or weight transducers could be used as well. When the target mass M of metal 34 has been aspired, for example 2500 kg, the compressed air supply 24 is closed with main compressed air directional valve 28 and the crucible 20 is moved with the crane 38 to receive molten metal from additional electrolytic cells until the crucible 20 is filled to capacity, typically with an 8500 kg load of molten metal.
A target metal flow rate Q of metal being siphoned and actual metal flow rate q are monitored and controlled by a control unit which may include a programmable logic controller (PLC) 40 and control loop(s) which may be integrated into the crane controls (not shown) for the crane 38. Conveniently, the PLC 40 may be associated with a monitor to display the instantaneous metal flow rate q for observation by an operator.
The PLC 40, as shown in
A vacuum air regulator 48 controls the flow of compressed air through compressed air line 26 which supplies air ejector 22. The regulator 48 comprises an adjustable secondary compressed air control valve assembly 50. The regulator 48 has a valve actuator 49 for adjusting the secondary compressed air control valve assembly 50 in response to the vacuum feedback pressure in line 44 and a vacuum set-point V fed to the regulator 48 from the PLC 40 through an electrical current-to-pressure (I/P) converter 53 coupled to compressed air line 51.
As illustrated by the screen print shown in
It will be appreciated that during this first stage (i), the PLC 40 is not fully activated and does not yet control the metal flow rate q.
In a second stage of siphoning (ii) having a typical duration ranging from 1 to 2 minutes, the vacuum set-point V is gradually increased by the PLC 40 until the instantaneous metal flow rate q (a calculated value) of metal being siphoned into the crucible 20 reaches a predetermined fraction of the initial target metal flow rate Q. The initial target flow rate Q is lower than the steady state target flow rate Q to compensate for an actual flow rate q that inevitably overshoots the target flow rate Q. At this time, the feedback loop from the crucible 20 through the pressure transducer 46 to the PLC 40 to control the regulator 48 is fully activated.
In a third stage of siphoning (iii), the fluid flow rate of the motive fluid through the ejector 22 and, thus, the vacuum set point V is continuously adjusted so that the metal flow rate q is fully controlled by the PLC 40 control loop(s) to correspond to the target metal flow rate Q. In this manner, small adjustments are made to the actual vacuum v being applied to the crucible 20. During this third stage of siphoning (iii), the target metal flow rate Q is progressively raised up to the steady state flow target (for instance about 9 Kg/sec).
It will be observed from
In a fourth and final stage (iv) of the siphoning process, the instantaneous metal flow rate q is adjusted to match the steady state target metal flow rate Q and siphoning is stopped when the metal mass m in crucible 20 reaches the target mass M, usually after an interval of about 5 minutes from the beginning of the second stage of siphoning.
The operation of the regulator 48 and associated valve assembly 50 will now be described with reference to a specific embodiment developed for this application which is illustrated in
The secondary compressed air control valve assembly 50 has a valve housing or body defining a primary compressed air inlet 52 and compressed air outlet 54. The control of air through the valve assembly 50 is regulated by a valve member 56 shown in
The body of the vacuum chamber 64 is bolted to the body of a pressure chamber 72 disposed below the vacuum chamber 64. The first flexible diaphragm 68 extends across the bottom of vacuum chamber 64 and is sealed between the vacuum chamber 64 and the pressure chamber 72. A second diaphragm 74 extends across the bottom of the pressure chamber 72 and is sealed between the pressure chamber 72 and a vented chamber 80. The pressure chamber 72 has a compressed air inlet 78 which, in use, is operatively coupled to the compressed air line 51 (see
The vented chamber 80 is open to atmospheric pressure through an air filter (not shown).
In operation, the position of valve stem 60 will fluctuate to control compressed air exiting compressed air outlet 54 thereby regulating the vacuum induced in the crucible headspace 30. The displacement of the diaphragms 68 and 74 in turn controls the position of the valve stem 60.
The rest position of the valve member 56 is the fully open position illustrated in
It will be appreciated that the effective surface area of the second diaphragm 74 is greater than the surface area presented by the first diaphragm 68. Accordingly, an increase in the air pressure admitted through compressed air inlet 78 to the pressure chamber 72, as controlled by I/P converter 53, will have a greater influence on the larger diaphragm 74 and operate to lower the stem 60 so as to open the valve assembly 50 and increase air flow through the valve assembly 50.
It will be understood that even a very weak vacuum in vacuum chamber 64 operates to pull upwardly on the valve stem 60 so as to close the valve member 56 whereas an increase in the air pressure admitted to the pressure chamber 72 will operate to depress the valve stem 60 downwardly so as to open the valve member 56, thereby requiring an even greater vacuum in order to close it.
The air pressure regulator 48 thus provides means to continuously adjust the target vacuum set-point V in response to real time changes in the instantaneous metal flow rate being siphoned. This is a vast improvement over prior art regulators which have predetermined target vacuum set points. With regulator 48, the system may be operated to siphon metal at a predefined metal flow rate and to change the steady state target metal flow rate Q according to prevailing operating conditions. Also with the PLC 40 and the regulator 48, it is possible to change a pre-defined metal flow rate to a different target, as needed, by providing a new target to the PLC 40.
While it has been proposed to determine the actual flow rate q of liquid being siphoned by measuring the weight of the crucible 20 and using the PLC 40 to process the weight data in order to determine the actual flow rate q, it is also contemplated to directly measure the liquid flow rate q by connecting a flow meter or the like to the siphoning tube 32.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2688730 | Dec 2009 | CA | national |
