Pulse-controlled metering device

Abstract: A pneumatic metering device for metering powdered materials, for example in connection with feeding aluminium oxide and aluminium fluoride to an electrolysis cell for production of aluminium, consisting of a container (1) in the form of a relatively long, closed channel or similar with a fluidizing element (4) fitted in the base for supply of air for fluidization of material which is located in the channel. The container is fitted with an inlet (2) for feeding material from a store in the form of a silo or similar and an outlet (3) for metered discharge of material from it. The area (10) below the inlet (2) is inactive and is not supplied with fluidizing air, whereas air is supplied intermittently to the fluidizing element (4) which is designed to cover essentially all other parts of the base.

Description: The present invention concerns a pneumatic dosage or metering device for metering powdered materials, for example in connection with feeding aluminium oxide and aluminium fluoride to an electrolysis cell for the production of aluminium, consisting of a container in the form of a relatively long, closed channel that is substantial horizontally oriented and provided with a fluidizing element fitted in the base for air supply for fluidization of material located in the channel. In addition, the container is fitted with an inlet for feeding material from a store in the form of a day tank, silo or similar and an outlet for metered discharge of material from the channel. There are previous devices of the above type in which transportation and metering take place in accordance with the fluidization principle. The prerequisite for being able to use such devices is that the material which is to be transported and metered is fluidizable. I.e. that it is pulverulent and has such granulometry and cohesion that the inlet air flow speed slowly forces decohesion between particles and a reduction of internal friction forces, causing the thus formed suspension to behave essentially as a homogenous fluid. Such materials include aluminium oxide used for smelting electrolysis as mentioned above, cements and plaster, slaked or unslaked lime, calcium fluoride, charger for plastic and rubber, catalysts, powdered carbon, sodium hydrogen-sulphate, phosphates, poly-phosphates, pyrophosphates, metal powder, pulverulent plastic materials, food products such as flour, milk powder, sugar, etc. The inventors have been working on the transportation and metering of fluidizable material for a number of years during which they have tried to find the simplest, most reasonably priced and most durable solutions in practice. In this connection the applicants have previously submitted two patent applications, Norwegian patent application no. 874220 for which a patent was granted under no. 162774 and Norwegian patent application no. 911178 corresponding to AU-B2-650 725 and EP-A-506 180. The first of these applications, no. 874220, concerns a metering device based on a type of volumetric metering in which, by means of fluidization, material flows from a store via a fluidizing channel to a fluidizing container where it is collected in a certain quantity, depending on the size of the container. When the container is full, the air supply to the fluidizing channel is stopped and air is supplied to the container so that the material in the container is fluidized and consequently discharged. This is, in fact, a simple solution which provides accurate doses but the size of the doses is difficult to adjust. The second of the applications, no. 911178, concerns a fluidizing channel for automatic level control for which an inactive zone, i.e. a zone which is not fluidized, is used in the area below the channel inlet for the fluidizable material. This channel cannot be used for metering purposes in the form in which it is shown and described in the application. On the basis of the experience gained by the inventors through long-term work with metering and transportation of fluidizable material, in accordance with the present invention they have designed a new metering device for such fluidizable material which is simpler than the metering device shown in the above-mentioned Norwegian patent application no. 874220, has a lower construction height and is more flexible regarding the possibility of adjusting the size of the individual dose. The invention is based on time-control of fluidizing air powder and is characterized in that an area inside the container under the inlet for feeding material to the container is inactive and is not supplied with fluidizing air, whereas air is supplied intermittently to the fluidizing element which is essentially designed to cover all other parts of the container base. According to one embodiment of the pneumatic metering device, the air supply to the channel may be controlled by an electronically controlled magnetic valve. The pneumatic metering device may further comprise a transverse, slidable restriction that is fitted in the channel. The present invention will now be described in further detail by means of examples and with reference to the attached figures in which Fig. 1 shows a pneumatic dosage or metering device in accordance with the present invention and Fig. 2 shows results plotted in a diagram following tests with the device shown in Fig. 1. As shown in Fig. 1, the metering device in accordance with the present invention consists in a container in the form of a long, closed channel 1 with an inlet 2 and an outlet 3 as well as a fluidizing element 4 fitted in the base. The fluidizing element may be detachable and of the type described in the applicant's own Norwegian patent application no. 920981 corresponding to AU-A-93 33 983, or it may be of a standard type with a canvas which is stretched out over the base at a distance from it and attached to the channel. As the figure shows, the area below the inlet 2 is not fitted with a fluidizing element and is thus not fluidized (the area is inactive). The aim of this is to prevent fluidization of material which is located in the overhead store, i.e. the tank 5. Tests have shown that in connection with fluidization of this area deviations occurred in the dose quantity metered which depended on the material quantity (the degree of filling) in the tank and metering inaccuracy increased considerably. Regarding the fluidizing element, pressure is supplied to this via an electronically controlled magnetic valve 6 via a pipe conduit 8 from a compressed-air store (not shown). The electronic control unit is programmable so that the material doses to be metered can be increased or decreased by increasing or reducing the pressure pulse length respectively and/or by varying the pressure pulse frequency. Even though it is not shown in the drawing, Fig. 1, the tank 5 is also fitted with a fluidizing element to fluidize material in the tank at regular intervals. This is necessary to ensure that material is transported towards the outlet 2. The electronic control unit also controls the valve (not shown) which opens for the air supply to the tank. It is important that this air supply to the tank does not coincide with the air supply to the channel. In such case material would flow from the tank through the channel, which would result in larger deviations in the doses metered. Moreover, the channel can be fitted with an air vent 7. However, as air consumption is very low during metering, this will normally not be required. In addition, a transverse, downward-stretching restriction 9, as shown in Fig. 1, is fitted above the end of the fluidizing element 4 next to the inlet 2. For practical purposes this may be slidable towards and away from the inlet 2 in order to adjust (optimize) the size of the doses. The main aim of this restriction is, however, to hold back the powder in the channel while the tank 5 is being refilled. The metering device in accordance with the present invention operates as follows: Material is filled in the tank 5 up to the level required. The electronic control unit is subsequently adjusted so that, for example, it meters out one dose per 20 seconds with a pulse length of 4 seconds and with fluidization of material in the tank between every 5th pulse. While the tank is being filled, the material will flow down into the channel 1 as shown in Fig. 1. When the control unit gives off a signal to the magnetic valve 6, the valve opens so that air is supplied to the channel. The material in the channel is subsequently fluidized and flows out through the outlet 3. At the same time new material will flow into the channel until 4 seconds have passed and the air supply is shut off. One dose has now been metered and additional doses will be metered in accordance with the same procedure every 20 seconds. A number of studies/tests have been carried out with the present invention in which a metering device has been used with a basic design as shown in Fig. 1. The result of one of these studies has been shown in further detail in Fig. 2. During this study the electronic control unit was set at a pulse of 4 seconds every 20 seconds, which was equivalent to a mean metered quantity of 0.625 kg per pressure pulse. As the figure shows, dose variation is very low with a standard deviation of 0.0069, equivalent to 1.1%. Air consumption is also very low, 1.16 m 3 /h. The present invention constitutes a new pneumatic metering device which is simpler and more reasonably priced than previous metering devices, and which has low air consumption, low construction height but at the same time great metering accuracy, independent of the filling level in the tank or store. In this connection it should be noted that the present invention as defined in the claims is not limited to a design in which the metering device is fitted under a tank as shown in Fig. 1. The metering device, or rather its inlet, can thus be attached to any kind of store in the form of a silo, a transport pipe, a channel or similar.

Claims: A pneumatic metering device for metering powdered materials, for example in connection with feeding aluminium oxide and aluminium fluoride to an electrolysis cell for production of aluminium, consisting in a container (1) in the form of a relatively long, closed channel or similar with a fluidizing element (4) fitted in the base for supply of air for fluidization of material which is located in the channel, the container being fitted with an inlet (2) for the supply of materials from a store in the form of a silo or similar and an outlet (3) for metered discharge of material from the container, characterized in that the area (10) below the inlet (2) is inactive and is not supplied with fluidizing air, whereas air is supplied intermittently to the fluidizing element (4) which is designed to cover essentially all other parts of the base. A pneumatic metering device in accordance with claim 1, characterized in that the air supply to the channel is controlled by an electronically controlled magnetic valve. A pneumatic metering device in accordance with claim 1, characterized in that a transverse, downward-stretching restriction (9) is fitted in the channel above the inactive area (10) and near the inlet (2). A pneumatic metering device in accordance with claim 3, characterized in that the restriction is slidable away from or towards the inlet (2).

Inventors: ENSTAD GISLE GOTFRED, NAGELL BERNT, OGNEDAL LEIV BORGE, PAULSEN KNUT ARNE, VIKSHAELAND BJORN HARALD, ENSTAD, GISLE GOTFRED, NAGELL, BERNT, OGNEDAL, LEIV BORGE, PAULSEN, KNUT ARNE, VIKSHAELAND, BJORN HARALD, Vikshäland, Bjorn Harald
Applicants: NORSK HYDRO AS, NORSK HYDRO ASA
Application year: 1993
Main classification: B65G69
IPC codes: C25C3, B65G53, G01F11, G01F13