The present invention relates to a method and means for feeding fluidisable materials. In particular, the invention relates to feeding of materials such as alumina and/or fluoride to aluminium electrolysis cells.

Traditionally almost all existing electrolysis technologies serving the aluminium production are equipped with silos and volumetric feeders and they are all controlled by volumetric measures. This exposes the process to the material density variations that typically can vary ± 10%. Furthermore, this variation combined with other storage and handling issues, such as segregation, may and does influence the feed stability to the pot negatively. Moreover, most electrolysis cells are today also already equipped with silos, hence the newly developed unit is based on the idea of retrofit.

In the past, several feeders for this type of material and application have been invented and patented. For instance, from EP 605037B1 there is known a feeder having a reservoir with an outlet downwards to a discharge channel having a fluidisable element in its bottom. The feeding is based upon a fluidised discharge that will be dependent upon the duration of the period of time the element is activated. Thus, the feeder works by the principle of time controlled discharge/dosage.

In accordance to the present invention there is now proposed a feeding device that has more accuracy than that of previous solutions. This is due to the fact that the feeder is provided with a device that isolates a discharge reservoir, from a main bulk of material (silo), in terms of forces. Thus, the said elements make the feeder less influenced by the actual filling level in the silo.

The flow of material with regard to activating the feed (start-up phase) and stopping the feed is also improved in the sense of repeatability.

This and further advantages can be achieved by the invention as defined by the accompanying claims.

In the following, the invention shall be further described by Figures and examples where:

Figure 1 discloses a feeding device in accordance with the present invention, seen in perspective, Figure 2 discloses a cut through a feeding device in accordance to Fig.1 and in a first cycle step where the silo of the feeder has been filled up,

Figure 3 discloses the feeding device of Fig. 2, in a second cycle step,

Figure 4 discloses the feeding device of Fig. 2, in a third cycle step,

Figure 5 is a diagram showing set point dose 1 kg AIF ₃ , measured versus predicted,

Figure 6 is a diagram showing set point dose 1 kg AI ₂ O ₃ , measured versus predicted.

Functional principle

As can be seen from Fig. 1 , the feeding device comprises a silo 1 with a fluidising element 3, such as one or more pads, in the bottom of the silo. The element can be activated by pressurised gas, such as air, flowing through a controllable valve at position 4 (not shown). The bottom of the silo and/or the element is slanted at an angle, preferably between 1 - 5°, towards an outlet of the silo.

At the outlet there is arranged a feeder reservoir, in the examples shown comprising a substantial vertical, downward converging part 5 shaped as a cone and a roof 6 as its main elements. Above the inlet to the cone 5 the roof 6 is arranged, and it will restrict material inside the silo to induce a pressure in the material in the cone. Further, the constructive details of the roof that is in contact with two side walls and one back wall of the silo and that have a projected area overlapping a part of the fluidising element, will be of high significance with regard to the stability of the feed from the silo.

The working principle of the roof together with the outlet and the fluidising element is that, due to the angle of repose (AR), the material will only be flowing into the cone when the element 3 is activated. When element 3 is non-activated, the transport of material to the inlet of the cone 5 is stopped and precisely defined by the material's angle of repose (AR).

From the cone 5 materials is led through a closed, slanted conveyer 11 having a fluidised element in its bottom (not shown). The pad can be activated by a valve at position 8 (not shown) allowing pressurised gas to be introduced in the element. Alumina and/or alumina fluoride is feed trough a hole 10 in the top of the silo 1 , when the silo is filled. When the silo 1 is filled with materials there will be introduced much air into the silo. This air needs to be evacuated, and assisted by the de-aeration tube 2 this excessive air from the inside of the silo will be ducted to the inside of the electrolysis cell (to be explained later). As shown in the Figure, the de-aeration tube 2 can be connected to the end part of the slanted conveyer 11 , above the material outlet tube 9.

Figure 2 illustrates the sequence when the silo is filled. The fluidising pad 3 is in-active when silo is filled, (i.e. valve in position 4 closed). Further, the details of the de-aeration tubes 2 and 7 are shown. The tubes end in the upper area of the silo. Further, in the Fig. 2 the angle of repose "AR" of the material "M" close to the outlet is shown, as well as the overhung "e" represented by the roof 6 and the extension of the fluidisable element 3. The fluidisable element preferably extends to the border of the inlet opening of the cone. In addition, the height of the roof is at level "h" above the fluidised element.

Preferably, the ratio between the overhung "e" and the height "h" is dependent on the materials angle of repose. For instance, if the angle of repose is 45°, the ratio e:h is preferably close 1 :1 or a little bit more to secure that there will be no transport of materials to the inlet opening when the fluidisable element is inactive.

It should also be understood that in the case there is applied a cone with a circular inlet, the roof may have a similar shape to establish an appropriate ratio e:h along the border of the inlet opening (not shown).

These parameters are decisive with regard to the ability the system will have to establish a defined lock i.e. an on/off function of the flow of powder material.

Figure 3 show the next sequence, i.e. the filling of the feeder reservoir that includes the volume of the cone 5 and the space beneath the roof 6. The valve marked 4 in Figure 1 is open, i.e. the fluidised element 3 is active. The volume of the feeder reservoir is defined by the cone 5 and the roof 6 in Figure 1. To evacuate the air during this sequence, the de- aeration tube 7 is utilised. The de-aeration tube 7 connects the space below the roof 6 with the upper part of the silo.

Fig. 4 discloses the dosing sequence, where time/capacity determined doses are discharged from dosage reservoir to the electrolysis cell. In this sequence the valve at position 8 is activated in a certain amount of time to give the wanted dose size, through the outlet 9. During this dosing the de-aeration of the fluidising air is vented via the silo and into the de-aeration tube 2 and trough the outlet tube 9 together with the material, and further into the cell. Further, the shape of the cone is of importance with regard to repeatability. For instance, the angle of the cone is of importance with regard to internal friction and flow of mass. The cone should have an angle that ensures that the frictional forces are unidirectional. A preferred value for the angle of the cone is 5-20° from the vertical, depending on the actual material.

The cone is preferably rotational symmetric along its length axis, and further having a circular cross-section. However, other shapes such as rectangular cross-section with inclined walls (tapered shape) may be applied.

As can be seen from the Figures 5 and 6, the repeatability of the dosages for aluminium fluoride and aluminium oxide respectively, are very accurate. The performance of the tests shows so low variation, that the accuracy of the scale used influenced the results.

An important fact to ensure the most accurate repeatability of the dosages is not to completely empty the feeder buffer. Thus the control of the sequences must comply with this requirement.

The feeder is preferably constructed in a manner allowing the whole volume to be quenched or flushed during a short period of time if necessary. This situation may occur in a circumstance with anode effects.